Modernize docs

[Nothing4You]

What do these changes do?

• use Python 3.11 for RTD build
• modernize build scripts by newly generating them with Sphinx 7 quickstart
• remove sphinxcontrib.asyncio extension, as it doesn't support Sphinx 6+ and we're not using any of its features anyway
• add sphinx_copybutton extension to make it easy to copy code snippets
• remove unused and broken (redirected to 2.0) SQLAlchemy intersphinx mapping
• switch to furo theme, which features a number of improvements, including Sphinx 7 compatibility
• fix examples/example_sa.py include in docs after it was renamed in 958fd0bada1c35d431097a4ccbb9210044636375
• fail RTD build if docs build generates warnings

This also depends on dropping Python 3.7, as Sphinx 3.6 dropped Python 3.7.

Are there changes in behavior for the user?

Only documentation.

Related issue number

• fixes #684
• closes #922

Checklist

• [x] I think the code is well written
• [x] Unit tests for the changes exist
• [x] Documentation reflects the changes
• [ ] Add a new news fragment into CHANGES.txt