docs: reorganize developer documentation and fix publishing CI

[PsiACE]

Summary

• Organize developer docs around SDK behavior and ACP contracts; preserve existing URLs.
• Publish independently from PyPI to gh-pages and request a Pages build.
• Keep PR checks read-only and reject broken local documentation links.

Validation

make check, strict docs build, 408 tests passed (1 skipped), actionlint, local links, stdio/HTTP2/WebSocket snippets, and local publication rehearsal.