Ignore github.com in linkcheck to keep the job a meaningful signal

GitHub rate-limits (429) the checker on nearly every run, so checking
its links produced a perpetually-red job. GitHub links rarely rot;
skipping them lets linkcheck flag only links it can actually verify.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: mariaob1201

_config.yml

@@ -69,10 +69,10 @@ sphinx:
     # usenix.org returns 403 to automated checkers (bot protection) but the
     # links resolve fine in a browser, so skip them in linkcheck.
     linkcheck_ignore:
+      # usenix.org returns 403 to automated checkers but works in-browser.
       - "https://www\\.usenix\\.org/.*"
-    # GitHub blob pages render heading anchors with client-side JS that
-    # linkcheck can't see; don't fail on missing anchors there.
-    linkcheck_anchors_ignore_for_url:
+      # GitHub rate-limits (429) the checker on nearly every run, which makes
+      # the result meaningless; GitHub links rarely rot, so skip them.
       - "https://github\\.com/.*"
     # Keep linkcheck fast: cap retries and how long we wait out a rate limit.
     linkcheck_timeout: 15