docs: retire Docs/ to the project wiki
The design, architecture, and test docs under Docs/ have been migrated to the
Forgejo wiki (the living reference). Remove them here and leave a tombstone
Docs/README.md mapping each old file to its wiki page.
Also gitignore the nested wiki working clone (pelagia-portal.wiki/), which is a
separate repo checked out beside this one.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
@
