Add documentation for envisage APIs

[aaronayres35]

fixes #302

This PR updates the module docstrings of api modules so that their content is documented.

The PR is currently flagged as WIP as I am not sure the best way to organize the classes etc so that the documentation is more clear (specifically in the main envisage.api module). ATM I have a Interfaces section and an Exceptions section, but I paused because I feel like this approach is not what we want long term. I imagine we would want to break it up into things like Plugins, ExtensionPoints and Services, or maybe have sections for Managers or Registrys. I am not sure, but will update this PR once this is decided on.

[rahulporuri]

I'm not sure exactly why but the built documentation is still empty.

[aaronayres35]

I'm not sure exactly why but the built documentation is still empty.

When I built it locally I was seeing it. Very strange, I will investigate

[aaronayres35]

I'm not sure exactly why but the built documentation is still empty.

When I built it locally I was seeing it. Very strange, I will investigate

I can't seem to reproduce. @kitchoi if you have the time would you be able to try building the docs and lmk if the api modules are populated?

[kitchoi]

Yep, I can try that.

[kitchoi]

I can also see the populated content for the api modules.
This is what I did:

$ git checkout doc-302-document-apis
$ python etstool.py install --editable
$ python etstool.py docs

Then open docs/build/index.html for the main page, and navigate to the corresponding api modules...
e.g.

[rahulporuri]

LGTM.

[rahulporuri]

I'm not sure exactly why but the built documentation is still empty.

Sorry about that. I always forget to install the package in editable mode.