feat: document docs generation process and improve missing docs UX

[cuihtlauac]

Summary

• Add "Automatic Documentation Generation" section to the /packages page explaining how package documentation is generated
• Improve the "documentation not found" error page with better guidance

Changes

/packages page

Added a new section explaining:

• How documentation is generated using odoc
• The role of ocaml-docs-ci in automatically building docs
• Brief mention of OCurrent as the underlying technology
• Tips for writing good documentation

Documentation not found page

• Changed title from "Page not found" to "Documentation not found" (more specific)
• Added explanation that docs are generated by ocaml-docs-ci
• Changed "Open an Issue" button to "Report Missing Docs" linking to ocaml-docs-ci issues
• Pre-fills the issue with package name and version for easier reporting

Screenshots

Would be helpful to add screenshots after testing locally.

Test plan

• Build locally with make build
• Visit /packages and verify the new section renders correctly
• Navigate to a package with missing docs and verify:
  • The error message is clear
  • The "Report Missing Docs" button opens a pre-filled GitHub issue

🤖 Generated with Claude Code

[sabine]

significant improvement

before	after