[BBS-282] [BBS-241] Update docs + Upload on RTD

[FrancescoCasalegno]

Fixes BBS-282, BBS-241.

Description

This PR makes our code ready for uploading on RTD.

1. Add info on installing with PyPI (i.e. pip install bluesearch) to the documentation and README.md.
2. Add a .readthedocs.yml configuration file for RTD.
3. Drop mysqlclient dependency (used only for mysqldb fast connections in our src code) if building on RTD. The rationale for this is as follows.
   1. mysqlclient needs the C library libmysqlclient-dev to be installed (see here)
   2. RTD does not allow to install C libraries (see here) and even though libmysqlclient-dev should be installed in the Docker image of RTD (see here) there is an known bug that makes this library unavailable (see here).
   3. Luckily we do not importthe mysqldb module anywhere in our code (it is only used for establishing efficient connections to the MySQL servers) so we can drop it from our dependencies if we just need to build the docs.
4. Important the stable build of RTD is failing. This is because the git commit we tagged with v0.1.0 still had mysqlclient as a dependency. For this reason, as this PR gets merged, we should then move the tag to this commit (I know, it's awful).

How to test?

Builds are passing and you can see the docs on RTD here.

Checklist

• This PR refers to an issue from the issue tracker.
  (if it is not the case, please create an issue first).
• Documentation and whatsnew.rst updated.
  (if needed)
• setup.py and requirements.txt updated with new dependencies.
  (if needed)
• All CI tests pass.

[jankrepl]

Looks great! Thanks for this PR.

[EmilieDel]

LGTM ! Thanks for the PR and the explanation!

[Stannislav]

Looking good, thanks!