[Docs] Migrate to mkdocs

[kozistr]

Problem (Why?)

related to #164

Solution (What/How?)

• migrate from sphinx to mkdocs
• fix docstring

Other changes (bug fixes, small refactors)

N/A

Notes

N/A

[codecov]

Codecov Report

Merging #206 (40e4c43) into main (d736c25) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #206   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           80        80           
  Lines         5732      5732           
=========================================
  Hits          5732      5732           

Files Changed	Coverage Δ
pytorch_optimizer/loss/jaccard.py	100.00% <ø> (ø)
pytorch_optimizer/lr_scheduler/linear_warmup.py	100.00% <ø> (ø)
pytorch_optimizer/optimizer/srmm.py	100.00% <ø> (ø)