Finish up sphinx domain

[klauer]

The sphinx domain allows for generating PLC project API documentation directly from the source code. Currently, it's pretty limited - the proof-of-concept from #12 is now in master.

A simple example might be:

conf.py

extensions = [
    "blark.sphinxdomain",
]

blark_projects = [
    "/Users/klauer/Repos/lcls-twincat-general/LCLSGeneral/LCLSGeneral.tsproj",
]
blark_signature_show_type = True

html_css_files = [
    "css/blark_default.css",
]

and the reStructuredText source:

.. bk:type:: ST_System

.. bk:function_block:: FB_TempSensor

.. bk:function_block:: FB_EtherCATDiag

    .. bk:variable_block:: VAR_OUTPUT

Where to from here? Some notes to self mostly:

• At the very least, I'd like to be able to document programs and other things I've forgotten.
• Would syntax highlighting be a possibility?
• What about dependency graphs?
• Multiple projects and dependencies (I had done an initial dependency store thing with whatrecord; https://github.com/pcdshub/whatrecord/blob/e0a5cbfaf62ae80df3aeb69565893a4efc806205/whatrecord/plugins/twincat_pytmc.py#L673; this needs overhauling)
• TMC file data structures?

[klauer]

GVLs and PROGRAM now work as of #19

[klauer]

Copying in part of my comment from #87 -

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)