docs(governance): introduce incubating subproject concept and update subproject list

[jackye1995]

See #5848 for more background and vote

[github-actions]

PR Review

This is a documentation-only governance update that introduces the "incubating subproject" concept. The changes are well-organized and clearly written.

Summary

The PR restructures the project hierarchy from a two-tier (core + subprojects) to a three-tier model (core + subprojects + incubating subprojects), moves several projects between tiers, and defines clearer graduation criteria.

No Critical Issues Found

This is a governance documentation change with no code impact. The changes are reasonable and provide a clearer path for experimental projects to mature.

Minor Observations (Non-blocking)

1. Removed graduation criteria: The old graduation path mentioned "Automated release mechanism" and "production use cases" — the new criteria are lighter ("Established use cases" without "production"). This seems intentional for the incubating→subproject path, but there's now no defined graduation path from subproject→core.

2. Committer privilege note: The note about committers losing privileges upon graduation could benefit from mentioning a timeline or transition period, though this may be intentionally left to PMC discretion.

LGTM — No blocking concerns.

[HaochengLIU]

lgtm. Feel free to merge it once u are comfortable to publish it

[westonpace]

Two minor grammar nits