Update actions variables documents (#30394) (#30405)

Backport #30394 by @lunny

Fix #30393

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Zettat123 <zettat123@gmail.com>

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

@@ -0,0 +1,41 @@
+---
+date: "2024-04-10T22:21:00+08:00"
+title: "Variables"
+slug: "actions-variables"
+sidebar_position: 25
+draft: false
+toc: false
+menu:
+  sidebar:
+    parent: "actions"
+    name: "Variables"
+    sidebar_position: 25
+    identifier: "actions-variables"
+---
+
+## Variables
+
+You can create configuration variables on the user, organization and repository level.
+The level of the variable depends on where you created it. When creating a variable, the
+key will be converted to uppercase. You need use uppercase on the yaml file.
+
+### Naming conventions
+
+The following rules apply to variable names:
+
+- Variable names can only contain alphanumeric characters (`[a-z]`, `[A-Z]`, `[0-9]`) or underscores (`_`). Spaces are not allowed.
+- Variable names must not start with the `GITHUB_` and `GITEA_` prefix.
+- Variable names must not start with a number.
+- Variable names are case-insensitive.
+- Variable names must be unique at the level they are created at.
+- Variable names must not be `CI`.
+
+### Using variable
+
+After creating configuration variables, they will be automatically filled in the `vars` context.
+They can be accessed through expressions like `${{ vars.VARIABLE_NAME }}` in the workflow.
+
+### Precedence
+
+If a variable with the same name exists at multiple levels, the variable at the lowest level takes precedence:
+A repository variable will always be chosen over an organization/user variable.