GeoTIFF release-notes draft in CHANGELOG (#2345 PR 5)

[brendancol]

Part of epic #2345.

Goal

Draft the GeoTIFF section of the release notes so the upcoming release ships with an accurate written support contract.

Scope

Add an [Unreleased] section to CHANGELOG.md (or extend the existing one) covering:

• Stable: which GeoTIFF paths are promoted to stable in this release, with one-line statements.
• Advanced: paths that are usable but not stable, with the rationale.
• Experimental: paths that exist but carry no parity claims (GPU, LERC, JPEG2000 / J2K, LZ4, etc.).
• Internal-only: paths that exist but are not externally interoperable.
• Unsupported for this release: combinations that fail closed (warped VRT, nested VRT, mixed-CRS VRT, cog=True, tiled=False, etc.).

Each bullet should link to the epic or PR that locked the behavior: #2286, #2321, #2340, #2341, #2342, #2344.

Out of scope

• Choosing the release version number. Leave the section as [Unreleased].
• Editing release docs outside CHANGELOG.md.

Acceptance

• CHANGELOG.md has an [Unreleased] GeoTIFF section with the five tier groupings.
• Each bullet matches SUPPORTED_FEATURES and the reference page.
• Each bullet links to its owning epic or PR.
• No bullet promises behavior outside the contract.