Change Sphinx documentation theme from Readthedocs to Furo

[hakonanes]

Description of the change

• Change Sphinx documentation theme from Readthedocs to Furo
• Remove old logo from v0.1
• Remove custom CSS used for the Readthedocs theme
• Add DefDAP and pycotem to related projects in the documentation. Close Should list pycotem in the list of related software that might be useful to users of kikuchipy #370.

Progress of the PR

• Docstrings for all functions
• Unit tests with pytest for all lines
• Clean code style by running black via pre-commit

For reviewers

• The PR title is short, concise, and will make sense 1 year later.
• New functions are imported in corresponding __init__.py.
• New features, API changes, and deprecations are mentioned in the
  unreleased section in doc/changelog.rst.

[hakonanes]

@onatlandsmyr and @friedkitteh (or anyone else who feel like it!), could you give your thoughts, if you have any, on how the documentation looks in the furo theme vs. the Readthedocs theme?

[friedkitteh]

@onatlandsmyr and @friedkitteh (or anyone else who feel like it!), could you give your thoughts, if you have any, on how the documentation looks in the furo theme vs. the Readthedocs theme?

In general I think the furo theme is a lot easier on the eyes. However, for plots it becomes worse (imo) as the title of the plots blends with the background e.g.

compared to the Readthedocs equivalent

Another example:

[hakonanes]

Thanks for the quick feedback, @friedkitteh! Yeah, Furo uses your browser's (OS') preferred light or dark theme. I have light enabled, so this is no problem for me, while I assume you have dark enabled, and so it is a problem. Apparently, we cannot at the moment force either mode (pradyunsg/furo#24).

A solution could be to add a white background to all headers, perhaps... I think it is worth doing this to start using the Furo theme.

[hakonanes]

By adding this to the Sphinx configuration file conf.py:

nbsphinx_execute_arguments = [
    "--InlineBackend.rc=figure.facecolor='w'",
]

and using

>>> print(s.axes_manager)
<Axes manager, axes: (3, 3|60, 60)>
            Name |   size |  index |  offset |   scale |  units
================ | ====== | ====== | ======= | ======= | ======
               x |      3 |      0 |       0 |     1.5 |     um
               y |      3 |      0 |       0 |     1.5 |     um
---------------- | ------ | ------ | ------- | ------- | ------
              dx |     60 |        |       0 |       1 |     um
              dy |     60 |        |       0 |       1 |     um

in user guide notebook cells instead of just s.axes_manager, I think the documentation looks OK against a dark background.

[hakonanes]

Will merge after checks pass. Thanks for the feedback @friedkitteh and @onatlandsmyr!