Modify and improve colors in style sheet for user documentation

[jessica-mitchell]

This PR modifies the CSS to change the color/highlighting of different items on ReadtheDocs.
These changes remove the default blue theme color that crept in our docs in some places, and tries to improve the clarity by removing background colors.

It also hides the entire name of python objects (so instead of nest.lib.hl_api_simulation.Simulate() being shown only Simulate() is visible) in the PyNEST API.

• Only python classes, functions, and methods have a background highlight, and the top border is changed for a left border
• Parameters, Returns, Raises etc have no highlighting (See https://nest-test.readthedocs.io/en/fix-css-docs/ref_material/pynest_apis.html)
• RestructuredText definition lists and glossary also have no color highlighting (See section Run NEST server: https://nest-test.readthedocs.io/en/fix-css-docs/nest_server.html)
• There are currently 4 colors used for admonition titles, based off of the NEST orange:
  • Note and See Also use a light yellowy orange
  • Warnings use NEST orange (At top of every page)
  • Danger uses red (See https://nest-test.readthedocs.io/en/fix-css-docs/nest_server.html)
  • Custom admonitions use dark gray (See https://nest-test.readthedocs.io/en/fix-css-docs/guides/random_numbers.html#)

Of course, these changes can be discussed and modified.

Resolves #2031

[jougs]

I'm happy with these changes and will merge them.