Documentation / poetry improvements and fixes

[sjoerd-bouma]

I suppose technically this could have been two PRs but no functionality should be affected.

1. Optional dependencies weren't really implemented correctly in pyproject.toml (I accept full responsibility). This made very little difference, but the advantage is they can now be installed also using the conventional pip syntax pip install nuradiomc[documentation, proposal] to install the correct optional dependencies. Previously, optional dependencies were implemented as developer dependencies, and would never be installed during pip installation.
2. The rest is minor fixes to the documentation: some missing docstrings from the recent refactoring, and an attempt to somewhat tidy up the new 'overview of times' page, though I find the content somewhat unclear still. The table of contents on code documentation pages now also only shows top-level submodule and subpackage names instead of every method of every class inside them, which I think improves clarity,

[fschlueter]

This isnt this wrong? This module only changes the trace start time but does no rolling.