docs: add actions to cheat sheet

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -1290,6 +1290,40 @@ PROXY_URL = socks://127.0.0.1:1080
 PROXY_HOSTS = *.github.com
 ```
 
+## Actions (`actions`)
+
+- `ENABLED`: **false**: Enable/Disable actions capabilities
+- `DEFAULT_ACTIONS_URL`: **https://gitea.com**: Default address to get action plugins, e.g. the default value means downloading from "https://gitea.com/actions/checkout" for "uses: actions/checkout@v3"
+
+`DEFAULT_ACTIONS_URL` indicates where should we find the relative path action plugin. i.e. when use an action in a workflow file like
+
+```yaml
+name: versions
+on:
+  push:
+    branches:
+      - main
+      - releases/*
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+```
+
+Now we need to know how to get actions/checkout, this configuration is the default git server to get it. That means we will get the repository via git clone ${DEFAULT_ACTIONS_URL}/actions/checkout and fetch tag v3.
+
+To help people who don't want to mirror these actions in their git instances, the default value is https://gitea.com
+To help people run actions totally in their network, they can change the value and copy all necessary action repositories into their git server.
+
+Of course we should support the form in future PRs like
+
+```yaml
+steps:
+  - uses: gitea.com/actions/checkout@v3
+```
+although Github don't support this form.
+
 ## Other (`other`)
 
 - `SHOW_FOOTER_BRANDING`: **false**: Show Gitea branding in the footer.