Reference docs gap-fill in geotiff.rst (#2345 PR 2)

[brendancol]

Part of epic #2345.

Goal

Audit docs/source/reference/geotiff.rst against the #2345 acceptance criteria and fill the remaining gaps so the reference page covers every promised dimension of the release contract.

Scope

Add or extend the following sections in docs/source/reference/geotiff.rst:

• Nodata lifecycle. Brief summary plus a cross-link to attrs_contract.rst for the full lifecycle. Make integer, float, NaN, and masked_nodata behavior explicit.
• Rotated and sheared transforms. State the read posture, the write posture, and which combinations fail closed.
• Remote read safety limits. Env vars and parameters that govern byte budgets, range coalescing, and private-host rejection. Reference the corresponding tests where they exist.
• GPU support. A short, explicit statement that GPU paths are experimental, what `SUPPORTED_FEATURES` reports for them, and what users should and should not expect.
• Known unsupported combinations. A short matrix or bulleted list that names combinations which fail closed today (e.g. `cog=True, tiled=False`, warped VRT, nested VRT, mixed-CRS VRT) and links to the regression test that locks each one.

Out of scope

• Code changes to xrspatial/geotiff/.
• Reworking the existing tier table or stable-COG section.

Acceptance

• All five sections above exist and are reachable from the page TOC.
• Each section either links to or names the regression test that backs it.
• The page does not contradict the existing tier table, SUPPORTED_FEATURES, or release_gate_geotiff.rst.
• Sphinx build succeeds without new warnings.