Create Index of Verilog Simulation Models

[rw1nkler]

This PR creates the index of Verilog Simulation Models with additional diagrams and code listings.

[rw1nkler]

Generally we don't use docs/source and just put the stuff under docs?

Fixed

Additionally, due to conflicts in conda dependencies, I created a separate conda environment for the documentation.

[mithro]

Any idea why CI is failing?

[mithro]

@rw1nkler -- Can you rebase this -- I think it is mostly ready to merge?

[rw1nkler]

This RTD failure is suspicious:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/symbiflow-arch-defs/conda/1566/lib/python3.8/site-packages/sphinx/cmd/build.py", line 276, in build_main
    app = Sphinx(args.sourcedir, args.confdir, args.outputdir,
  File "/home/docs/checkouts/readthedocs.org/user_builds/symbiflow-arch-defs/conda/1566/lib/python3.8/site-packages/sphinx/application.py", line 244, in __init__
    self.setup_extension(extension)
  File "/home/docs/checkouts/readthedocs.org/user_builds/symbiflow-arch-defs/conda/1566/lib/python3.8/site-packages/sphinx/application.py", line 398, in setup_extension
    self.registry.load_extension(self, extname)
  File "/home/docs/checkouts/readthedocs.org/user_builds/symbiflow-arch-defs/conda/1566/lib/python3.8/site-packages/sphinx/registry.py", line 412, in load_extension
    raise ExtensionError(__('Could not import extension %s') % extname,
sphinx.errors.ExtensionError: Could not import extension sphinxcontrib.images (exception: No module named 'sphinxcontrib.images')

Extension error:
Could not import extension sphinxcontrib.images (exception: No module named 'sphinxcontrib.images')

I see that the sphinxcontrib-images are correctly installed in the conda environment:

Collecting sphinxcontrib-images
  Downloading sphinxcontrib_images-0.9.2-py2.py3-none-any.whl (63 kB)

Yet they are not accessible during build stage. I have to investigate what is going on there.