Swap docs to MkDocs

[schlegelp]

This PR switches from Sphinx to MkDocs Material. A preview is already visible here: https://navis-org.github.io/navis/

Pretty happy with the results but there were a couple snags along the way:

1. Tutorials:
   With Sphinx we used NB convert to generated reST documents from the Jupyter notebooks. Here we switched to mkdocs-gallery - a port of Sphinx gallery - which uses .py Python scripts (with the # %% cell syntax). Disadvantage: to get cell outputs (such as figures) we need to actually execute the script during the build. The issue with this is that (a) it takes a while (not too bad though) and (b) some example that e.g. need to download data or have a long run time (like the NBLASTs) had to be made static. Advantage: smaller file size and the build process effectively doubles as tests. mkdocs-gallery works nice for the actual tutorials. For rendering other documents such as the quickstart.md I used markdown-exec.
2. Autodocs
   Sphinx had the nice autodoc functions (e.g. the :: autosummary directive) which automatically generate the html files for the docstrings. With MkDocs we have to do a bit more leg work. I ended up using mkdocs-gen-files but I had to write a custom script to collect the functions (see gen_ref_pages2.py). In addition, I used mkdocs-macros-plugin to write macro that lets me write e.g. {{ autosummary("navis.BaseNeuron.bbox") }} (see main.py) to replace the Sphinx ::autosummary directive.

Additional changes:

• TreeNeuron.soma_pos can now be set to None to remove any soma
• navis.Volume now has a .units property similar to neurons
• removed neuprint.fetch_skeleton from navis' neuprint interface to avoid confusion with our own fetch_skeletons (plural!)
• removed navis.screenshot() (may bring that back later) - use Viewer.screenshot instead
• implement NeuronList.__or__ magic method
• new method: NeuronList.get_neuron_attributes
• new function: navis.graph.skeleton_adjacency_matrix()
• Neuron.pre/postsynapses is now case-insensitive when looking for the respective connectortype
• plot2d/3d: alpha can now be given as a list/array with an alpha value for each neuron
• fixed the MICrONs interface includes new functions and improvements
• fixed a bug with TreeNeuron.simple where a soma could potentially lead to a non-branch, non-leaf node be retained
• fixed a couple issues in plot2d and pl3d

TODOs:

• add some missing gallery thumbnails
• fix the ecosystem graphic
• use markdown-exec to add some plots to quickstart (see this example; use # markdown-exec: hide to hide lines of code)
• add tutorial for skeleton plotting
• installation: move optional dependencies into a separate document?
• add some eyecandy to landing page?
• remove TOC from landing page?
• edit the card icons on landing page
• remove TOC from tutorials (should make them wider):

  ---
  hide:
    - navigation
    - toc
  ---
  

  This will require a fix in mkdocs-gallery though (see Defining page setup in README.md smarie/mkdocs-gallery#96)

[schlegelp]

Old and new tests are finally passing and I ticked off most TODOs.

The workflow for building & deploying docs hasn't actually run yet because it is only triggered when pushing to the main branch (which makes sense, I guess). I'm guessing I forgot to add some of the mkdocs plugins to the requirements but I will cross that bridge when we get there.

I think at this point I will go ahead and merge.