docs: fix missing api reference for Emitter

[tigarmo]

The docs were missing because of the __all__ attribute in messages.py. To get around this, craft a special purpose rst file to be used when documenting that module, explicitly asking autodoc to generate docs for Emitter and EmitterMode.

Fixes #208

• [ ] Have you followed the guidelines for contributing?
• [ ] Have you signed the CLA?
• [ ] Have you successfully run tox?

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (ce2ee08) 94.76% compared to head (cc0899c) 94.77%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #212 +/- ## ======================================= Coverage 94.76% 94.77% ======================================= Files 7 7 Lines 1070 1072 +2 Branches 197 198 +1 ======================================= + Hits 1014 1016 +2 Misses 53 53 Partials 3 3 ```

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