Add GPlately 2.1.0 release checklist

[Copilot]

Creates a living draft checklist (release/release-notes-2.1.0.md) to track everything needed before the 2.1.0 release can be announced.

Contents

• Completed (from 01 prepare for release 2 1 0 #419): Raster reconstruction perf tuning, PlateReconstruction.shallow_copy(), Raster.from_points(), enhanced Raster.copy() and fill_value handling
• Pending code/features: Seafloor gridding PRs (Improve seed point reconstruction in seafloor gridding #413, Deform continental polygons in seafloor gridding #414 — both milestone-tagged 2.1, still open/draft), regional raster reconstruction (Support regional raster reconstruction #417, open design questions), plot sub-folder reorganisation (Create a new sub-folder "plot" #409), pickling KeyError fix (PlateModel.__getstate__ raises KeyError: 'executor' during pickling #386), notebook updates (Update example notebooks in Notebooks/Examples #420/420 update example notebooks in notebooksexamples #421)
• Pre-release mechanics: USING_DEV_VERSION flag, version/tag, Docker image version file, DEV warning suppression
• Validation: pytest, notebooks, examples, pip/conda on all three platforms, Docker, CLI commands
• Docs: gh-pages folder, online doc audit, PyPI page review

• Fixes Things need to be done for the coming GPlately release 2.1.0 #422