FIX: make sphinx domain work again

[klauer]

(This is part of a larger branch which got to be unmanageably large - breaking it up into separate PRs)

This PR aims to get the sphinx domain working again. It's been broken since at least the solution-loading refactor.

For those anxious for Sphinx support, consider sphinx-plc. I think their implementation has better sphinx integration overall that blark may borrow ideas from in the future.

Future work will be, probably roughly in this order:

1. Investigate how to better structure the domain and directives (and learn from the Python implementation, sphinx-plc, etc.)
2. Bring my doc testing repo to GitHub so that others can more easily try blark's sphinx support
3. Rework/rewrite it in a submodule, breaking up the pieces a bit more (blark.sphinxdomain.* perhaps)

Well, more on this later...

Related: #17

[codecov-commenter]

Codecov Report

Merging #87 (d9b34fc) into master (e950984) will increase coverage by 0.0%. The diff coverage is 0.0%.

@@          Coverage Diff           @@
##           master     #87   +/-   ##
======================================
  Coverage    80.6%   80.6%           
======================================
  Files          26      26           
  Lines        5559    5570   +11     
======================================
+ Hits         4484    4493    +9     
- Misses       1075    1077    +2     

Files	Coverage Δ
blark/sphinxdomain.py	0.0% <0.0%> (ø)

... and 1 file with indirect coverage changes