Remove v1 and v2 specification

[sanketverma1704]

Hi everyone. 👋🏻

Following this from https://github.com/zarr-developers/zarr-specs/pull/276, I removed the v1 and v2 specification from https://zarr.readthedocs.io/en/stable/spec.html.

For now, I've added this message for redirecting:

The V1/2/3 Specification has been migrated to its website → https://zarr-specs.readthedocs.io/.

Please note: We can add specific redirects for individual pages; see this: https://docs.readthedocs.io/en/stable/user-defined-redirects.html#user-defined-redirects.

User-defined redirects can be added via the readthedocs.org dashboard.

TODO:

• [ ] Changes documented in docs/release.rst
• [ ] GitHub Actions have all passed
• [ ] Test coverage is 100% (Codecov passes)