Last remaining relevant todo notes have been filed as issues in GitLab.

Based on comments by Jeroen in issue #14.

This is quite a large commit, touching many things related to developer
documentation. It is all focussed on getting as much of this as possible
into the new Sphinx-based documentation.

Some highlights:

- Start Sphinx-based developer documentation, including fairly complete
  instructions for installation and configuration.
- Remove epydoc API docs.
- Rework some docstrings to conform to reStructuredText, so they can be
  used in the API docs generated by Sphinx.
- Move all of the top-level text files to reStructuredText so they can
  linked from the Sphinx-based docs and for consistency.
- Remove many obsolete things from the extras/ directory, including old
  installation scripts and migrations.

Many of the installation related documentation and scripts are removed
or adapted in light of the new automated deployment using Ansible.