[MAINT] Build docs and automate publishing when a PR is merged

[rbavery]

we will need to confirm that readthedocs is set up correctly to build with merged PRs: https://docs.readthedocs.io/en/stable/tutorial/

Currently for PRs open to merge into this branch https://github.com/CosmiQ/solaris/tree/0.5.0, readthedocs is not being run to build docs, this might be because the PR is not being opened to merge into master

CC @srmsoumya @rodrigoalmeida94

[rodrigoalmeida94]

For the docs Readthedocs is essentially a separate service managed in it's own a web page. In order to trigger docs build per PR we would need to change settings there. I'll send an email to the maintainer to ask for access to that as well.