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

[westonpace]

Minor nit

Suggested change

	| Release a new stable major version of core project | 3 | PMC | GitHub Discussions | 1 week |
	| Release a new stable minor version of core project | 3 | PMC | GitHub Discussions | 3 days |
	| Release a new stable patch version of core project | 3 | PMC | GitHub Discussions | N/A |
	| Release a new stable major version of the core project | 3 | PMC | GitHub Discussions | 1 week |
	| Release a new stable minor version of the core project | 3 | PMC | GitHub Discussions | 3 days |
	| Release a new stable patch version of the core project | 3 | PMC | GitHub Discussions | N/A |

[westonpace]

Suggested change

	| Code modifications in core project (except changes to format specifications) | 1 (excluding proposer) | Maintainers with write access | GitHub PR | N/A |
	| Code modifications in the core project (except changes to format specifications) | 1 (excluding proposer) | Maintainers with write access | GitHub PR | N/A |