chore: check for invalid markdown links in docs weekly (#6795)

* chore(ci): add an automatic check to verify markdown links. 

As per #6793, there are multiple broken links in our docs. This action will check if all markdown links are valid.

cc: @ammario @bpmct

* Revert "chore(ci): add an automatic check to verify markdown links. "

This reverts commit 294767e056.

* Create cron-weekly.yaml

* add workflow dispatch for testing

* fmt

* Update cron-weekly.yaml

* remove slack notification for now

* Add slack notification

* Update cron-weekly.yaml

* Add logs URL

* fmt

.github/workflows/cron-weekly.yaml

@@ -0,0 +1,30 @@
+name: Markdown Links Check
+# runs every monday at 9 am
+on:
+  schedule:
+    - cron: "0 9 * * 1"
+  workflow_dispatch: # allows to run manually for testing
+
+jobs:
+  check-docs:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@master
+
+      - name: Check Markdown links
+        uses: gaurav-nelson/github-action-markdown-link-check@v1
+        id: markdown-link-check
+        # checks all markdown files from /docs including all subfolders
+        with:
+          use-quiet-mode: "yes"
+          use-verbose-mode: "yes"
+          config-file: ".github/workflows/markdown.links.config.json"
+          folder-path: "docs/"
+
+      - name: Send Slack notification
+        run: |
+          curl -X POST -H 'Content-type: application/json' -d '{"msg":"Broken links found in the documentation. Please check the logs at ${{ env.LOGS_URL }}"}' ${{ secrets.DOCS_LINK_SLACK_WEBHOOK }}
+          echo "Sent Slack notification"
+        env:
+          LOGS_URL: ${{ https://github.com/coder/coder/actions/runs/${{ github.run_id }}