Separate documentation files for separate features

David's idea to remove redundancy between namespace docs and the top-level readme while (still) enabling comprehensive documentation:

- Create separate (markdown) files for the different library features/sublibraries.
- Link to those in the top-level readme (instead of the current slight clutter).
- Reduce namespace docstrings and also link to above documentation.

Slightly related: create a changelog that we can use to document breaking changes instead of those being the first thing you see here.