build(docs): Twitter follow link returns 403 during audit

Observed failure

The Documentation workflow's Build site job fails during the external link audit because Twitter returns HTTP 403 for the Orleans follow link:

Link audit found 1 issue(s):
- https://twitter.com/intent/follow?screen_name=msftorleans (index.yml:224): returned 403; add a reasoned exact-URL allowlist entry only if the target cannot be probed reliably.

Evidence

The identical failure occurs on both an unrelated pull request and current main:

• PR #11348 run: https://github.com/dotnet/orleans/actions/runs/36112079394/job/107997600414
• Scheduled main run: https://github.com/dotnet/orleans/actions/runs/36116777906/job/108012625129
• Current source: docs/site/src/content/docs/index.yml:224

All source builds and test/provider jobs pass on #11348, so this failure is independent of its TestingHost changes.

Expected outcome

Make the link audit deterministic for this URL, either by replacing the link with a reliably probeable destination or adding a reasoned exact-URL allowlist entry if Twitter intentionally rejects automated probes.