New docs, with sphinx + markdown

[tony]

Like #266 / new_docs, but with sphinx + markdown

Also adds manifest.json

todo

• Note to add meta tags: https://stackoverflow.com/a/43154489
• Add something to autogenerate manifest.json, via __about__.py
• perhaps add something to autoresize/generate images based on a 512png
• Find if its possible to read from pyproject.toml

[codecov]

Codecov Report

Merging #267 into master will increase coverage by 0.01%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #267      +/-   ##
==========================================
+ Coverage   85.57%   85.59%   +0.01%     
==========================================
  Files          16       16              
  Lines         804      805       +1     
==========================================
+ Hits          688      689       +1     
  Misses        116      116              

Impacted Files	Coverage Δ
libvcs/exc.py	100.00% <ø> (ø)
libvcs/git.py	68.94% <ø> (ø)
libvcs/hg.py	100.00% <ø> (ø)
libvcs/svn.py	68.42% <ø> (ø)
setup.py	0.00% <0.00%> (ø)
libvcs/base.py	100.00% <100.00%> (ø)
libvcs/shortcuts.py	100.00% <100.00%> (ø)
libvcs/util.py	95.31% <100.00%> (ø)

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