[TASK] Add concise README with workflow status

Replace Documentation/ folder with a concise README that shows:
- Workflow table with real-world adoption status
- Backport secrets documentation (APP_ID, APP_PRIVATE_KEY)
- Usage example and migration guide

Input/secret details live in the YAML workflow files themselves.

Signed-off-by: Sebastian Mendel <info@sebastianmendel.de>

Author: CybotTM

README.md

@@ -1,2 +1,56 @@
-# TYPO3 Documentation Team
-Official TYPO3 Documentation
+# TYPO3 Documentation — Shared Workflows
+
+Reusable GitHub Actions workflows for the
+[TYPO3 Documentation](https://github.com/TYPO3-Documentation) organization.
+
+Workflows placed here are automatically available to every repository in the
+organization without allow-list configuration.
+
+## Workflows
+
+| Workflow | Status | Purpose |
+|----------|--------|---------|
+| `reusable-backport.yml` | Production (8 repos) | Backport PRs via labels |
+| `reusable-test-documentation.yml` | Available | Documentation rendering test |
+| `reusable-apply-precommit.yml` | Available | Auto-fix whitespace via pre-commit |
+| `reusable-docs-render.yml` | Available | Render docs with typo3-docs-theme or guides-cli |
+| `reusable-php-quality.yml` | Available | CS Fixer + PHPStan + XML lint |
+| `reusable-php-tests.yml` | Available | PHP unit/integration test matrix |
+| `reusable-php-command.yml` | Available | Generic PHP + Composer command |
+
+**Production** = actively used and proven stable across multiple repos.
+**Available** = on main, ready to adopt, not yet used in production.
+
+Each workflow file contains full input/secret documentation in its YAML
+`inputs:` and `secrets:` definitions.
+
+## Usage
+
+```yaml
+jobs:
+  backport:
+    uses: TYPO3-Documentation/.github/.github/workflows/reusable-backport.yml@main
+    with:
+      label_pattern: "^backport ([^ ]+)$"
+    secrets:
+      APP_ID: ${{ secrets.APP_ID }}
+      APP_PRIVATE_KEY: ${{ secrets.APP_PRIVATE_KEY }}
+```
+
+### Secrets (backport)
+
+| Secret | Required | Description |
+|--------|----------|-------------|
+| `APP_ID` | no | GitHub App ID for creating backport PRs |
+| `APP_PRIVATE_KEY` | no | GitHub App private key (PEM format) |
+
+PRs created with the default `GITHUB_TOKEN` do not trigger CI workflows on the
+backport PR. Passing a GitHub App token ensures backport PRs trigger their own CI
+checks. Falls back to `GITHUB_TOKEN` if neither secret is provided.
+
+## Migration
+
+1. Identify which shared workflow matches your inline job.
+2. Replace the `jobs:` section with a `uses:` call.
+3. Pass inputs via `with:` and secrets via `secrets:`.
+4. Remove inline `permissions:`, `steps:`, and `runs-on:`.