Improve documentation

- Provide summary of each action on main page
- Extract detailed documentation for each action
- Document usage with forked repositories

setup-gradle/README.md

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+# The `setup-gradle` action
+
+This action is a simple wrapper around `gradle/gradle-build-action`, removing the deprecated `arguments` parameter (and thus removing the ability to _execute_ gradle).
+The intention is to eventually deprecate `gradle-build-action` with this being the replacement.
+
+### Example usage
+
+```yaml
+name: Build
+
+on:
+  workflow_dispatch:
+  push:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout sources
+      uses: actions/checkout@v4
+    - name: Setup Gradle
+      uses: gradle/actions/setup-gradle@v0
+    - name: Build with Gradle
+      run: ./gradlew build
+```
+
+See the [`gradle-build-action` documentation](https://github.com/gradle/gradle-build-action/blob/main/README.md) for a full description of this action.