Re-add docs/CNAME after js.org PR #11845 merges

[EthanThatOneKid]

Re-add CNAME after js.org PR merges

The CNAME file was removed to avoid GitHub Pages redirecting to linked-markdown.js.org before DNS is configured.

Once js-org/js.org#11845 merges, run:

echo "linked-markdown.js.org" > docs/CNAME
git add docs/CNAME
git commit -m "Add CNAME for linked-markdown.js.org"
git push

This will let GitHub Pages serve the custom domain once js.org DNS points to GitHub Pages.