Add annotations to the backend code

[confluence]

The backend code is currently not adequately documented. We should add annotations to all classes and functions which describe their interfaces and their intended function. If we use a format compatible with an automatic documentation generator like Doxygen or Sphinx, we will be able to use it to produce developer-facing documentation for the codebase automatically.

This is an epic; we should make smaller issues for adding annotations to more specific areas of the code and for setting up the automatic generation (which includes deciding on a tool and a way to host the docs).

[confluence]

Here's a possible direction (which would allow us to use Sphinx and Read The Docs, which we already use for other components and should also be able to use for the frontend).

[confluence]

Here is a similar project. We may be able to do something simpler; the preamble lists some alternatives.