Improve publish readiness and group HexDocs guides

[pcharbon70]

Summary

• update ExDoc config to include explicit guide group sections:
  • User Guides
  • Developer Guides
• set docs main page to Jido.Document for stable docs entrypoint
• clean documentation references so mix docs runs without warnings

Why

PR #18 was already merged; these follow-up publish-readiness/doc-config changes are separated into their own PR.

Validation

• mix docs
• mix quality
• mix hex.build
• mix hex.publish --dry-run (stops at expected Hex auth gate on local machine)