Clean up release notes in preparation for v3

[dstansby]

This cleans out the release notes page, in preparation for v3. I decided to:

• Remove all the previous release notes for v1 and v2. These will get out of sync with new minor releases of v2, so I thought better to remove them and link out to the v2 branch for these.
• Remove the existing v3 release notes. I think these dried up at some point, so instead of having an imcomplete set I thought better to remove them and link out to the v3 migration guide, and then start a normal release note service once v3 is released.

Thoughts?

[normanrz]

Sounds like a good idea. I also stumbled across this issue because some old release notes link to sections in the docs that now describe very different behavior.

[jhamman]

+1 on doing this. Can we get this in tomorrow?

[jhamman]

One small request but otherwise looks good.

[jhamman]

Let's seed the release note section for 3.0.1

.. release_3.0.1:

3.0.1 (unreleased)
------------------

...

[dstansby]

We shouldn't have the 3.0.1 title in the 3.0.0 release, so shouldn't we hold off here until after release?

[jhamman]

We typically seed the release notes with the next release so contributors have a spot to start adding release notes. We can do this right after the 3.0.0 release I suppose.

[normanrz]

In numcodecs, we also seed a section but without version number. https://github.com/zarr-developers/numcodecs/blob/main/docs/release.rst?plain=1#L16

[dstansby]

That's after a release is made, the notes for the current release don't have an unreleased section: https://numcodecs.readthedocs.io/en/stable/release.html