Generate gazelle documentation and publish it to RTD

[aignas]

Right now the docs are not present for the macros that we export and it would be great to have docs generated for all of the publicly available macros/rules for gazelle.

Possible alternatives:

1. Use rules_python RTD domain.
2. Use a rules_python_gazelle_plugin RTD domain.
3. Keep the same README.

If we choose 1., then we may need to add rules_python_gazelle_plugin as a dev_dependency in order to get the distribution filegroup in order to be able to build the docs. This may be a little bit hairy due to circular dependencies?

If we choose 2., then we would need to register a new domain and use sphinxdocs from rules_python_gazelle_plugin workspace and it would give us some dog-fooding opportunities for the sphinxdocs rules.

If we choose 3., then we potentially need to point people to the README and the source and it has to be our policy for some time.