fix(docs): update Discord invite URL

[bkearns]

Executive Summary

• Replace the expired Discord invite across every published docs page.
• Update the shared Asciidoctor header so regenerated docs retain the correct invite.
• Use https://discord.gg/he2uw2Vpgy consistently.

Verification

• No stale fxz9kNJ5a references remain.
• Docs regeneration with Asciidoctor 2.0.20 has zero content drift.
• scripts/check-site.py passes.
• git diff --check passes.