Experiment with Furo theme for PyRTL's documentation.

[fdxmw]

Some nice Furo features:

• Full on-this-page method index (right sidebar)
• Edit-this-page and dark mode buttons (top of page)

This commit also enables two extensions:

• sphinx_autodoc_typehints, which fetches argument and return type information from type hints, rather than specifying them again in :param: or :rtype:
• sphinx_copybutton, which adds a copy button to all code samples

This commit disables these extensions, which don't seem to do anything:

• hoverxref
• extlinks
• intersphinx

[codecov-commenter]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (9c86d42) 90.92% compared to head (763bdba) 90.92%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #436 +/- ## ============================================ Coverage 90.92% 90.92% ============================================ Files 24 24 Lines 6139 6139 ============================================ Hits 5582 5582 Misses 557 557 ```

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