Add GitHub Action for automated markdown link checking

Implements automated link checking to prevent broken documentation links.
Uses markdown-link-check action that runs on:
- Push to master (when .md files change)
- Pull requests (when .md files change)
- Weekly schedule (Mondays at 8am UTC)
- Manual workflow dispatch

Includes configuration to:
- Ignore localhost and mailto links
- Handle GitHub relative links properly
- Retry on rate limiting
- Check all markdown files in docs/ folder

This addresses the need for automated 404 detection in documentation
as requested by Justin.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ihabadham

.github/markdown-link-check-config.json

@@ -0,0 +1,32 @@
+{
+  "ignorePatterns": [
+    {
+      "pattern": "^http://localhost"
+    },
+    {
+      "pattern": "^https://localhost"
+    },
+    {
+      "pattern": "^mailto:"
+    }
+  ],
+  "replacementPatterns": [
+    {
+      "pattern": "^/",
+      "replacement": "https://github.com/shakacode/react_on_rails/blob/master/"
+    }
+  ],
+  "httpHeaders": [
+    {
+      "urls": ["https://docs.github.com", "https://github.com"],
+      "headers": {
+        "Accept": "text/html"
+      }
+    }
+  ],
+  "timeout": "20s",
+  "retryOn429": true,
+  "retryCount": 3,
+  "fallbackRetryDelay": "30s",
+  "aliveStatusCodes": [200, 206]
+}

.github/workflows/check-markdown-links.yml

@@ -0,0 +1,34 @@
+name: Check Markdown Links
+
+on:
+  push:
+    branches: [master]
+    paths:
+      - '**.md'
+      - '.github/workflows/check-markdown-links.yml'
+  pull_request:
+    paths:
+      - '**.md'
+      - '.github/workflows/check-markdown-links.yml'
+  schedule:
+    # Run weekly on Monday at 8am UTC
+    - cron: '0 8 * * 1'
+  workflow_dispatch:
+
+jobs:
+  markdown-link-check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Check markdown links
+        uses: gaurav-nelson/github-action-markdown-link-check@v1
+        with:
+          use-quiet-mode: 'yes'
+          use-verbose-mode: 'no'
+          config-file: '.github/markdown-link-check-config.json'
+          folder-path: 'docs/'
+          file-extension: '.md'
+          max-depth: -1
+          check-modified-files-only: 'no'
+          base-branch: 'master'