Reworked documentation and formalize test suite

[Helveg]

This PR addresses 2 issues:

• The test suite was not documented, and its dependencies not specified
• The documentation was minimal, and hard to extend to a multi document structure, or to add new information that does not belong in the dashing module docstring.

Test suite

• Add test extra dependencies and add them to the dev extra dependencies.
• Make the pytest suite exit as gracefully as possible with a failing exit code on Windows.
  • pexpect does not support spawn, and the alternative cross-platform API pexpect.spawn_popen.SpawnPopen doesn't seem to support enough features for our use case

Documentation

• Add docs extras and add them to the dev extra dependencies.
• Decouple the documentation from the dashing module docstring, while maintaining it without duplication for the Overview section
• Add a Developers section with the test suite docs
• Add a Getting Started guide (still needs work to continue beyond the basics, e.g., example of each family of tiles and how to make your own tile)
• Add autosummary to generate a full API reference, that wouldn't need to be maintained if we add new submodules or restructure the package.
• Switch theme to furo, I personally find it less visually cluttering than alabaster, and it plays together nicely with sphinx-design (not added yet) if we want to start including some more modern/Material UI in the docs like tabs, code blocks, badges, buttons, cards, ...

[Helveg]

This is to prevent a missing static path '_static' error during build

[FedericoCeratto]

Thanks for the excellent contribution!