Add summaries to API docs

[tbekolay]

Motivation and context:
@celiasmith pointed out that the API docs pages are long and hard to navigate. This PR adds summaries to frontend_api and networks to (hopefully) make them easier to navigate.

When built, networks looks like this:

And frontend_api looks like this:

That's only the amount that would fit on the screen, it goes on for a while more.

How has this been tested?
Built the docs to see how it renders.

How long should this take to review?

• Quick (less than 40 lines changed or changes are straightforward)

Types of changes:

• Non-code change (touches things like tests, documentation, build scripts)

Checklist:

• I have read the CONTRIBUTING.rst document.
• I have updated the documentation accordingly.
• I have included a changelog entry.
• [na] I have added tests to cover my changes.
• All new and existing tests passed.

Still to do:

One thing we should discuss is whether the frontend_api summary should contain all things that are listed on that page, or if we should omit things that users aren't likely to interact with very much, like base classes and LearningRule, Thoughts?

[jgosmann]

Is it possible to exclude all the distributions from that table and have a separate table only for distributions?

[tbekolay]

Is it possible to exclude all the distributions from that table and have a separate table only for distributions?

Yep, all is possible. I can split it up so there's a summary per heading too, that might make sense (since the headings are shown in the left sidebar and are clickable to get to subgroups easily).

[jgosmann]

Yep, all is possible.

That would be great!

[tbekolay]

Pushed a fixup separating the frontend_api summaries into separate sections; here's the distributions section (the others look similar):

This seems like an improvement to me 👍