DO NOT MERGE: verify the site/ deprecation notice

[jwhartley]

Throwaway PR verifying the changes in #3384. Based on that branch, not master, so both the new workflow and the new CODEOWNERS file are in play.

Checking three things:

1. The rewritten sticky comment posts and reads correctly to a contributor.
2. No new pr-preview/ directory appears on gh-pages.
3. CODEOWNERS auto-requests review for a site/ change.

Closing as soon as those are confirmed.

[github-actions]

site/ is the deprecated docs location

This PR touches site/, which is no longer the source for
docs.estuary.dev. The site is now built from two other repos:

• estuary/docs for platform docs:
  guides, concepts, reference, getting started.
• estuary/connectors for
  connector pages, under docs/reference/Connectors/.

If you meant to change the docs, the change will not reach the site
from here. Please move it to whichever of those two repos owns the
page.

If this is a code PR that happens to include a doc file, the code
part is fine. One thing to know: site/ PRs now need review from
the docs owners before they can merge, so the fastest way through is
to move the doc edit to the repo above, or drop it from this PR.

There are no PR previews for site/ any more. The old
docs.estuary.dev/pr-preview/pr-N/ links all return 404.

site/ is kept until 2026-09-30 (tentative) so it can serve as a
rollback path for the new site, then it is removed. The build check
on this PR only confirms that rollback target still compiles.

[jwhartley]

Verified, closing.

1. Sticky comment posted and reads correctly. It uses the same sticky header as the old one, so existing site/ PRs will have their dead preview link replaced by this notice on their next push.
2. No new pr-preview/ directory on gh-pages. Count unchanged at 574.
3. CODEOWNERS auto-requested @aeluce. Sorry for the notification, Emily, this was the test firing.
4. Build check passed in 2m24s under its new job name, so both workflow files parse.