GH Actions: add a workflow to automatically deploy the wiki

This commit introduces a workflow, which will:
* Automatically deploy the wiki files to the [PHP_CodeSniffer repo wiki](https://github.com/PHPCSStandards/PHP_CodeSniffer/wiki) on every push to the `main` branch.
* Will do a dry-run, without actually deploying whenever a PR is created which would update the wiki files.

This way, the wiki is opened up to contributions via pull requests and is no longer limited to only edits made by me.

This commit also adds a `_Footer.md` file, which will automatically be displayed at the bottom of each wiki page to point out that the wiki is editable via PRs to this repo.

Notes:
* Wiki commits will get the same commit message as the _last_ commit on the `main` branch.
    For that reason, merge commits are not allowed in this repo and PRs with only one commit are strongly preferred.
* If the net effect of a commit results in no changes to the wiki files (CI changes and such), no commit will be made to the wiki.
* The workflow is set up to fail if GitHub has an outage for git operations.
    That should protect the wiki from going down by a broken/partial commit and allows for retriggering a deploy once the outage has passed by re-running the failed build.

Future scope (upcoming):
* Automate generation of the Table of Contents for wiki pages.
* Automate re-generation of output examples used in the wiki pages.

Refs:
* https://github.com/Andrew-Chen-Wang/github-wiki-action
* https://github.com/crazy-max/ghaction-github-status

Author: jrfnl

.github/workflows/publish-wiki.yml

@@ -0,0 +1,56 @@
+name: Publish wiki
+on:
+  push:
+    branches:
+      - 'main'
+    paths:
+      - wiki/**
+      - .github/workflows/publish-wiki.yml
+  # Do a dry-run (check, no deploy) for PRs.
+  pull_request:
+    paths:
+      - wiki/**
+      - .github/workflows/publish-wiki.yml
+  # Allow running this workflow manually from the Actions tab.
+  workflow_dispatch:
+  # Allow this workflow to be triggered from outside.
+
+# Allow only one concurrent deployment, skipping runs queued between the run in-progress and latest queued.
+# However, do NOT cancel in-progress runs as we want to allow these production deployments to complete.
+concurrency:
+  group: "publish-wiki"
+  cancel-in-progress: false
+
+jobs:
+  publish-wiki:
+    name: "Publish Wiki"
+    if: github.repository == 'PHPCSStandards/PHP_CodeSniffer-documentation'
+
+    runs-on: ubuntu-latest
+
+    permissions:
+      # Needed for the commit to the wiki.
+      contents: write
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Check GitHub Git Operations status
+        uses: crazy-max/ghaction-github-status@v4
+        with:
+          git_threshold: partial_outage
+
+      - name: Deploy to wiki
+        uses: Andrew-Chen-Wang/[email protected]
+        env:
+          COMMIT_MSG: ${{ github.event.head_commit.message }}
+        with:
+          strategy: 'clone'
+          path: 'wiki/'
+          commit-message: ${{ env.COMMIT_MSG }}
+          repository: PHPCSStandards/PHP_CodeSniffer
+          token: ${{ secrets.PHPCS_PUSH_TO_WIKI_TOKEN }}
+          dry-run: ${{ github.event_name == 'pull_request' }}
+          disable-empty-commits: true
+          preprocess: false

wiki/_Footer.md

@@ -0,0 +1,3 @@
+_Found a mistake ? Think this documentation can be improved ?_
+
+Contributions to this wiki are welcome! Submit a pull request to the [Documentation repository](https://github.com/PHPCSStandards/PHP_CodeSniffer-documentation) to propose your changes.