Docusaurus-ify (#26051)

This PR cleans up the docs in a way to make them simpler to ingest by
our [docs repo](https://gitea.com/gitea/gitea-docusaurus).

1. It includes all of the sed invocations our ingestion did, removing
the need to do it at build time.
2. It replaces the shortcode variable replacement method with
`@variable@` style, simply for easier sed invocations when required.
3. It removes unused files and moves the docs up a level as cleanup.

---------

Signed-off-by: jolheiser <john.olheiser@gmail.com>

docs/content/usage/actions/comparison.en-us.md

@@ -0,0 +1,167 @@
+---
+date: "2023-04-27T15:00:00+08:00"
+title: "Compared to GitHub Actions"
+slug: "comparison"
+sidebar_position: 30
+draft: false
+toc: false
+menu:
+  sidebar:
+    parent: "actions"
+    name: "Comparison"
+    sidebar_position: 30
+    identifier: "actions-comparison"
+---
+
+# Compared to GitHub Actions
+
+Even though Gitea Actions is designed to be compatible with GitHub Actions, there are some differences between them.
+
+## Additional features
+
+### Absolute action URLs
+
+Gitea Actions supports defining actions via absolute URL, which means that you can use actions from any git repository.
+Like `uses: https://github.com/actions/checkout@v3` or `uses: http://your_gitea.com/owner/repo@branch`.
+
+### Actions written in Go
+
+Gitea Actions supports writing actions in Go.
+See [Creating Go Actions](https://blog.gitea.com/creating-go-actions/).
+
+## Unsupported workflows syntax
+
+### `concurrency`
+
+It's used to run a single job at a time.
+See [Using concurrency](https://docs.github.com/en/actions/using-jobs/using-concurrency).
+
+It's ignored by Gitea Actions now.
+
+### `run-name`
+
+The name for workflow runs generated from the workflow.
+See [Workflow syntax for GitHub Actions](https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#run-name).
+
+It's ignored by Gitea Actions now.
+
+### `permissions` and `jobs.<job_id>.permissions`
+
+See [Workflow syntax for GitHub Actions](https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#permissions).
+
+It's ignored by Gitea Actions now.
+
+### `jobs.<job_id>.timeout-minutes`
+
+See [Workflow syntax for GitHub Actions](https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idtimeout-minutes).
+
+It's ignored by Gitea Actions now.
+
+### `jobs.<job_id>.continue-on-error`
+
+See [Workflow syntax for GitHub Actions](https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idcontinue-on-error).
+
+It's ignored by Gitea Actions now.
+
+### `jobs.<job_id>.environment`
+
+See [Workflow syntax for GitHub Actions](https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idenvironment).
+
+It's ignored by Gitea Actions now.
+
+### Complex `runs-on`
+
+See [Workflow syntax for GitHub Actions](https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idruns-on).
+
+Gitea Actions only supports `runs-on: xyz` or `runs-on: [xyz]` now.
+
+### `workflow_dispatch`
+
+See [Workflow syntax for GitHub Actions](https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onworkflow_dispatch).
+
+It's ignored by Gitea Actions now.
+
+### `hashFiles` expression
+
+See [Expressions](https://docs.github.com/en/actions/learn-github-actions/expressions#hashfiles)
+
+Gitea Actions doesn't support it now, if you use it, the result will always be empty string.
+
+As a workaround, you can use [go-hashfiles](https://gitea.com/actions/go-hashfiles) instead.
+
+## Missing features
+
+### Variables
+
+See [Variables](https://docs.github.com/en/actions/learn-github-actions/variables).
+
+It's under development.
+
+### Problem Matchers
+
+Problem Matchers are a way to scan the output of actions for a specified regex pattern and surface that information prominently in the UI.
+See [Problem matchers](https://github.com/actions/toolkit/blob/main/docs/problem-matchers.md).
+
+It's ignored by Gitea Actions now.
+
+### Create an error annotation
+
+See [Creating an annotation for an error](https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#example-creating-an-annotation-for-an-error)
+
+It's ignored by Gitea Actions now.
+
+## Missing UI features
+
+### Pre and Post steps
+
+Pre and Post steps don't have their own section in the job log user interface.
+
+## Different behavior
+
+### Downloading actions
+
+Gitea Actions doesn't download actions from GitHub by default.
+"By default" means that you don't specify the host in the `uses` field, like `uses: actions/checkout@v3`.
+As a contrast, `uses: https://github.com/actions/checkout@v3` has specified host.
+
+The missing host will be filled with `https://gitea.com` if you don't configure it.
+That means `uses: actions/checkout@v3` will download the action from [gitea.com/actions/checkout](https://gitea.com/actions/checkout), instead of [github.com/actions/checkout](https://github.com/actions/checkout).
+
+As mentioned, it's configurable.
+If you want your runners to download actions from GitHub or your own Gitea instance by default, you can configure it by setting `[actions].DEFAULT_ACTIONS_URL`. See [Configuration Cheat Sheet](administration/config-cheat-sheet.md#actions-actions).
+
+### Context availability
+
+Context availability is not checked, so you can use the env context on more places.
+See [Context availability](https://docs.github.com/en/actions/learn-github-actions/contexts#context-availability).
+
+## Known issues
+
+### `docker/build-push-action@v4`
+
+See [act_runner#119](https://gitea.com/gitea/act_runner/issues/119#issuecomment-738294).
+
+`ACTIONS_RUNTIME_TOKEN` is a random string in Gitea Actions, not a JWT.
+But the `docker/build-push-action@v4` tries to parse the token as JWT and doesn't handle the error, so the job fails.
+
+There are two workarounds:
+
+Set the `ACTIONS_RUNTIME_TOKEN` to empty manually, like:
+
+``` yml
+- name: Build and push
+  uses: docker/build-push-action@v4
+  env:
+    ACTIONS_RUNTIME_TOKEN: ''
+  with:
+...
+```
+
+The bug has been fixed in a newer [commit](https://gitea.com/docker/build-push-action/commit/d8823bfaed2a82c6f5d4799a2f8e86173c461aba?style=split&whitespace=show-all#diff-1af9a5bdf96ddff3a2f3427ed520b7005e9564ad), but it has not been released. So you could use the latest version by specifying the branch name, like:
+
+``` yml
+- name: Build and push
+  uses: docker/build-push-action@master
+  with:
+...
+```