Update docs: PyData sphinx theme, simplify autogen of API

[hakonanes]

Description of the change

• Change docs theme from Furo to PyData's theme, in line with kikuchipy, orix and HyperSpy (soon pyxem as well)
• Simplify autogeneration of API by listing all public facing functionality in module's __all__ lists. We can thus remove the hard-coded list in reference.rst. The new lists define public functionality.
• Add dev dependency on sphinx_design for nice cards and tabs on the landing page
• Use pyxem logo until a new one is made

Progress of the PR

• [n/a] Docstrings for all functions
• [x] Unit tests with pytest for all lines
• [x] Clean code style by running black

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 CHANGELOG.rst.
• [ ] Contributor(s) are listed correctly in credits in diffsims/release_info.py and in .zenodo.json.

[hakonanes]

Let's merge #194 and release v0.5.2 before merging this.

[hakonanes]

@pc494 and @CSSFrancis, could one of you review this PR? There aren't any breaking changes as I'm aware of.

[CSSFrancis]

@pc494 and @CSSFrancis, could one of you review this PR? There aren't any breaking changes as I'm aware of.

I'll try to get to this later today unless @pc494 beats me to it!

[hakonanes]

Thanks for having a look!