Update dates for HDMF release

[mavaylon1]

Summary of changes

• Update dates for hdmf release
• Add .readthedocs.yaml to build the docs. This was added to hdmf but was missed here.

PR checklist for schema changes

• Update the version string in docs/source/conf.py and common/namespace.yaml to the next version with the suffix "-alpha"
• Add a new section in the release notes for the new version with the date "Upcoming"
• Add release notes for the PR to docs/source/hdmf_common_release_notes.rst and/or
  docs/source/hdmf_experimental_release_notes.rst

[mavaylon1]

Review Notes:

1. Since this is minor/non-schema changes I did not create any of the steps from the checklist.
2. The yaml file specifies python 3.9. This is to match the one in hdmf, but I am not sure why we chose that given 3.8 is the minimum. I don't think it really matters, but I wanted to note it.

[rly]

Thanks and looks good to me!