[WIP] Update documentation "Why OT" section

[rflamary]

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Motivation and context / Related issue

How has this been tested (if it applies)

Checklist

• The documentation is up-to-date with the changes I made.
• I have read the CONTRIBUTING document.
• All tests passed, and additional code has been covered with new tests.

[agramfort]

@rflamary I just pushed to this branch many sphinx warnings fixes. There are still quite a lot.

Rather than commenting out sphinx-gallery just build the doc with :

make html-noplot

[codecov]

Codecov Report

Merging #220 (acd4927) into master (cb3e24a) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #220   +/-   ##
=======================================
  Coverage   92.40%   92.40%           
=======================================
  Files          16       16           
  Lines        3056     3056           
=======================================
  Hits         2824     2824           
  Misses        232      232           

[agramfort]

@rflamary can you look into these sphinx errors in quickstart.rst file?

/Users/alex/work/src/POT/docs/source/quickstart.rst:538: WARNING: 'any' reference target not found: ot.lp.barycenter
/Users/alex/work/src/POT/docs/source/quickstart.rst:562: WARNING: 'any' reference target not found: ot.lp.barycenter
/Users/alex/work/src/POT/docs/source/quickstart.rst:713: WARNING: 'any' reference target not found: ot.dr.wda
/Users/alex/work/src/POT/docs/source/quickstart.rst:713: WARNING: 'any' reference target not found: ot.dr.fda
/Users/alex/work/src/POT/docs/source/quickstart.rst:723: WARNING: 'any' reference target not found: ot.dr
/Users/alex/work/src/POT/docs/source/quickstart.rst:794: WARNING: 'any' reference target not found: ot.unbalanced.barycenter_unbalanced_sinkhorn_unbalanced
/Users/alex/work/src/POT/docs/source/quickstart.rst:850: WARNING: 'any' reference target not found: auto_examples/unbalanced-partial/plot_partial
/Users/alex/work/src/POT/docs/source/quickstart.rst:926: WARNING: 'any' reference target not found: ot.gpu.dist
/Users/alex/work/src/POT/docs/source/quickstart.rst:927: WARNING: 'any' reference target not found: ot.gpu.sinkhorn
/Users/alex/work/src/POT/docs/source/quickstart.rst:928: WARNING: 'any' reference target not found: ot.gpu.sinkhorn_lpl1_mm
/Users/alex/work/src/POT/docs/source/quickstart.rst:930: WARNING: 'any' reference target not found: ot.gpu.to_gpu
/Users/alex/work/src/POT/docs/source/quickstart.rst:930: WARNING: 'any' reference target not found: ot.gpu.to_np