Opensource doc creation

[MaxHalford]

I've tidied up (a bit) the tool that we use to parse our documentation and convert to it to an MkDocs friendly format. It's here. This PR updates our codebase accordingly.

I think this is a nice step forward because it will allow us (mostly me) to work on the tool independently and share it with others

[codecov-commenter]

Codecov Report

Merging #675 (9a78b8f) into master (d1948d0) will decrease coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #675      +/-   ##
==========================================
- Coverage   87.62%   87.44%   -0.19%     
==========================================
  Files         321      321              
  Lines       16265    16241      -24     
==========================================
- Hits        14253    14202      -51     
- Misses       2012     2039      +27     

Impacted Files	Coverage Δ
river/linear_model/test_glm.py	77.14% <0.00%> (-22.86%)	⬇️
river/tree/nodes/hatc_nodes.py	83.24% <0.00%> (-2.62%)	⬇️
river/metrics/mutual_info.py	96.39% <0.00%> (-0.91%)	⬇️
river/datasets/synth/sea.py	85.71% <0.00%> (-0.65%)	⬇️
river/datasets/synth/concept_drift_stream.py	63.63% <0.00%> (-0.65%)	⬇️
river/datasets/synth/sine.py	83.33% <0.00%> (-0.31%)	⬇️
river/datasets/synth/stagger.py	83.33% <0.00%> (-0.31%)	⬇️
river/utils/skmultiflow_utils.py	77.65% <0.00%> (-0.24%)	⬇️
river/datasets/synth/hyper_plane.py	92.30% <0.00%> (-0.20%)	⬇️
river/datasets/synth/led.py	94.54% <0.00%> (-0.20%)	⬇️
... and 12 more

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