docs: new documentation with mkdocs

[tlambert03]

I've very much enjoyed/preferred my experiences working with mkdocs, mkdocstrings, and the mkdocs-material theme. This PR converts the docs here to use mkdocs... it also dramatically reorganizes things to make usage patterns clearer;

edit ... really, this is a complete rewrite of pretty much every page 😂

[codecov]

Codecov Report

Base: 89.86% // Head: 89.65% // Decreases project coverage by -0.22% ⚠️

Coverage data is based on head (62e3bcf) compared to base (0bb5323).
Patch coverage: 93.44% of modified lines in pull request are covered.

❗ Current head 62e3bcf differs from pull request most recent head 9d58412. Consider uploading reports for the commit 9d58412 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #527      +/-   ##
==========================================
- Coverage   89.86%   89.65%   -0.22%     
==========================================
  Files          39       37       -2     
  Lines        4442     4417      -25     
==========================================
- Hits         3992     3960      -32     
- Misses        450      457       +7     

Impacted Files	Coverage Δ
src/magicgui/schema/_guiclass.py	96.66% <ø> (-1.12%)	⬇️
src/magicgui/tqdm.py	94.73% <ø> (ø)
src/magicgui/widgets/_table.py	96.14% <ø> (ø)
src/magicgui/widgets/bases/__init__.py	100.00% <ø> (ø)
src/magicgui/widgets/bases/_categorical_widget.py	80.28% <ø> (-4.23%)	⬇️
src/magicgui/widgets/bases/_create_widget.py	93.93% <ø> (ø)
src/magicgui/widgets/bases/_ranged_widget.py	89.14% <ø> (ø)
src/magicgui/widgets/bases/_slider_widget.py	96.87% <ø> (ø)
src/magicgui/widgets/protocols.py	100.00% <ø> (ø)
src/magicgui/_util.py	80.70% <90.90%> (+6.05%)	⬆️
... and 16 more

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[alisterburt]

It's amazing how good docs really make things crystallise in your mind - genuinely so psyched on guiclass and can't wait to use it :)

[TimMonko]

All the new documentation is SOOO much better! (wish it was here 2 months ago haha!) Thank you so so much for all the work put into this! The logic, the flow, the extent of detail. Even after all I've learned this year with magicgui, this documentation is so much easier to navigate, clears up some sticking points I had, and more. I won't be missing the old docs. Cheers!

[tlambert03]

Made my day, thanks for stopping by to leave a note