Enable v3 branch builds on readthedocs

[dstansby]

In the version selection at https://zarr.readthedocs.io/en/latest/ it looks like the v3 branch isn't enabled - it would be good to enable it so it's easy to see what the v3 docs look like as it's developed.

[jhamman]

We should be ready to get started on this once #1884 goes in.

[jhamman]

@zarr-developers/steering-council - can you activate builds on the v3 branch. (see also #1905)

[joshmoore]

Done. https://zarr.readthedocs.io/en/v3/ is now active though the version number looks odd.