Pin markdown-link-check action to specific commit SHA for security

Uses tcort/github-action-markdown-link-check@a800ad5f (v1.1.0)
instead of mutable version tag to prevent supply chain attacks.

Author: ihabadham

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

@@ -24,7 +24,7 @@ jobs:
       - uses: actions/checkout@v4
 
       - name: Check markdown links
-        uses: tcort/github-action-markdown-link-check@v1
+        uses: tcort/github-action-markdown-link-check@a800ad5f1c35bf61987946fd31c15726a1c9f2ba # v1.1.0
         with:
           use-quiet-mode: 'yes'
           use-verbose-mode: 'no'