Release checklist polish in release_gate_geotiff.rst (#2345 PR 3)

[brendancol]

Part of epic #2345.

Goal

Make docs/source/reference/release_gate_geotiff.rst usable as the maintainer-facing go / no-go gate for the GeoTIFF-focused release.

Scope

Audit the existing checklist and, for each promised tier row, make sure all of the following are present:

• One-line acceptance statement.
• Path to the regression test file (or files) that locks the row.
• A link to the epic that owns the row: Promote COG support to ready/stable with compliance and parity gates #2286, GeoTIFF release hardening: define and lock down supported VRT contract #2321, Epic: GeoTIFF release contract and feature tiering #2340, Epic: GeoTIFF correctness and backend parity release gate #2341, Epic: Conservative VRT support contract for GeoTIFF release #2342, Epic: GeoTIFF remote/source safety hardening #2344.
• Tier annotation that matches SUPPORTED_FEATURES.

Then add a short "how a maintainer runs this gate" section at the top:

• The pytest selection or marker that runs the gated tests.
• What to do when a row is intentionally skipped (e.g. GPU unavailable).
• The decision rule for promoting or demoting a tier based on gate output.

Out of scope

• Adding new tests. If a gate row points at a missing test, file a follow-up rather than writing it here.
• Rewriting the tier table in geotiff.rst.

Acceptance

• Every gate row has acceptance + test path + epic link + tier.
• The new "how to run" section is present at the top.
• A maintainer can decide ready / not ready by walking the checklist top to bottom.
• Sphinx build succeeds without new warnings.