[MRG] Add logo to POT

[rflamary]

Types of changes

This PR adds a logo to the documentation and the code to generate this logo to the repository.

There are two versions of the logo for clear/white background and dark background.

The colored white background version is:

The dark background logo in the documentation:

Motivation and context / Related issue

How has this been tested (if it applies)

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

Merging #357 (f5681cf) into master (b47d504) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #357      +/-   ##
==========================================
- Coverage   93.63%   93.60%   -0.04%     
==========================================
  Files          22       22              
  Lines        5595     5595              
==========================================
- Hits         5239     5237       -2     
- Misses        356      358       +2