Update docs in `main` for cell type release

[sjspielman]

Purpose

This PR contains all changes necessary for the cell type docs release. Given the current in-progress AnnData release in #208, this PR was created by branching off of allyhawkins/anndata-release and merging in development, as no cherry-picking was necessary.

This PR is being filed as a draft to main, since that is the ultimate target branch. In order to mark it ready for formal review and de-draft, these changes will first need to be made:

• Merge in main after #208 is merged; this will provide the AnnData release CHANGELOG date and keep us in sync with main.
• Update the CHANGELOG with this cell type release date (note that I'm leaving the associated issue #209 open until this is finalized)

Issue addressed

• CHANGELOG issue: #209
• Release issue: #210

Checklist for publishing to main

• [x] This branch contains all commits/content ready to be released.
• [x] The most recent CHANGELOG entry includes a date. If this release required a CHANGELOG entry, it should be today's date.

[allyhawkins]

Cell typing appears to be on the portal, so we should get these docs updates in. I added today's date to the changelog and this is now ready for review.