docs(community): document GitHub Sponsors tiers, recognition and SPONSORS.md

[lusoris]

Summary

GitHub Sponsors is live, and this PR documents it. SPONSORS.md is the single source for the tier table and the sponsor list; the new docs page "Support VMAFx" includes both from it (pymdownx.snippets) instead of repeating them. README, GOVERNANCE section 8 and the docs navigation now point at them, and ADR-2689 records the decision (recognition only, four monthly tiers, USD on GitHub, EUR on Ko-fi, Patreon coming).

Type

• feat — new feature
• fix — bug fix
• perf — performance improvement
• refactor — no behavior change
• docs — documentation only
• test — test-only
• build / ci — tooling / infra
• port — cherry-pick from upstream Netflix/vmaf
• sycl / cuda / simd — backend-specific

Checklist

• Commits follow Conventional Commits (the commit-msg hook enforces this).
• Every commit is signed off (git commit -s).
• make format && make lint is green locally. make lint-md, make docs-fragments-check and the strict docs build (make docs-build) pass.
• Unit tests pass — not applicable: documentation only.
• SIMD/GPU cross-backend check — not applicable: no code path touched.
• Feature extractor twins — not applicable: none touched.
• License header on new source files — not applicable: no source files added.
• Breaking change — not applicable: none.
• The ADR row lives in docs/adr/_index_fragments/2689-sponsorship-tiers-recognition-only.md; docs/adr/README.md and _order.txt are untouched.

Bug-status hygiene (ADR-0165)

• no state delta: documentation of a funding channel, no bug opened or closed.

Netflix golden-data gate (ADR-0024)

• I did not modify any assertAlmostEqual(...) score in the Netflix golden Python tests.
• No golden value needs to change.

Deep-dive deliverables (ADR-0108)

• Research digest — no digest needed: trivial, the tier facts come from the maintainer's decisions.
• Decision matrix — in ADR-2689 ## Alternatives considered.
• AGENTS.md invariant note — no rebase-sensitive invariants.
• Reproducer / smoke-test command — below.
• CHANGELOG fragment — changelog.d/added/sponsors-live.md.
• Rebase note — no rebase impact: fork-only community files.

Reproducer

make docs-fragments-check && make docs-build

Known follow-ups

• Add the Patreon link and a patreon: entry to .github/FUNDING.yml when the euro page is live.
• Place logos by hand when a Company or Lead sponsor sends one.

[lusoris]

Negative check for the SPONSORS.md includes (run locally, file restored afterwards): pointing line 35 of docs/support-vmafx.md at a missing file (SPONSORS-missing.md:tiers) or a missing section (SPONSORS.md:no-such-section) makes mkdocs build --strict exit 1; the second fails with SnippetMissingError: Snippet section 'no-such-section' could not be located. So a renamed section marker or a moved file breaks the docs build instead of silently dropping the sponsor list.