docs: document the v2 alpha migration

[uermel]

Summary

• document the exact initial copick, shared UI, reader, ChimeraX, Python, and Zarr alpha stack
• provide checksum-first GitHub wheel installation instead of an unqualified install path
• state that alpha bundles remain off the public ChimeraX Toolshed
• document legacy OME-Zarr 0.4/v2 readability and new OME-Zarr 0.5/v3 writes
• explain metadata-defined pyramid paths and layout-agnostic chunk/shard/key/codec handling
• update README, Toolshed copy, tutorial, and embedded ChimeraX help

Release Please will generate the alpha changelog entry when its release PR is created. Stable floor replacement, ecosystem promotion, and Toolshed submission remain gated on every published alpha passing together.

Stack

Layer 4 of 4. GitHub retains uermel/zarr-v3-x3-backend-gates as this PR's base because the PR belongs to a stack;
that branch is now commit-identical to uermel/zarr-v3-x2-volume-parity. The former backend-smoke content from #94
was withdrawn and removed.

Validation

• complete behavior suite passes at Zarr 3.1.6 and 3.3.0
• a ChimeraX Daily 1.13 build emits Requires-Python: >=3.14 and passes inspect_bundle.py
• source, tests, and scripts compile
• documentation does not claim native backend or published-candidate results that have not run

Part of #90
Part of #85