chore - generate RFC index from RFC files (#7)

[dannymeijer]

Summary

This PR makes docs/rfcs/README.md deterministic by generating the RFC table from the RFC markdown files themselves. It adds an update/check script, a focused CI workflow for stale-index detection, Make targets, and contributor docs for the RFC index workflow.

Type of change

• Bug fix
• New feature
• Refactor / maintenance
• Documentation
• CI / tooling
• RFC (adds/updates docs/rfcs/*)

Area(s)

Select the primary areas touched (labels sync from checked lines when the triage workflow runs):

• Package & tests
• Specification (RFCs)
• Documentation
• Automation & repo config
• Other

Key details

• User-facing behavior: Contributors can run make rfc-index after RFC header changes and make rfc-index-check before committing.
• Internals: scripts/generate_rfc_index.py parses each docs/rfcs/NNN_*.md file for its RFC number, title, and status, then rewrites only the marked generated region in docs/rfcs/README.md.
• Risks: The generated table now reflects actual RFC header status values, so stale hand-maintained rows changed as part of the first generation.

Testing / verification

• make ci (or make fmt-check, make build, make test)
• Manual verification described below

Manual verification notes:

• make rfc-index-check
• python3 -m py_compile scripts/generate_rfc_index.py
• YAML parse for .github/workflows/rfc-index.yml
• git diff --check HEAD^..HEAD

Docs impact

• No docs changes needed
• Docs updated
• Docs follow Divio intent (tutorial/how-to/reference/explanation) where applicable

If docs updated:

• Link(s): docs/rfcs/README.md, CONTRIBUTING.md

Checklist

• I kept public docs user-focused and moved internals to contributing docs when appropriate
• I avoided duplicating canonical install/run instructions in multiple places
• I added/updated tests where it materially reduces regressions

Closes #7