[WIP] Set dev version and add minigallery to quick start guide

[rflamary]

Types of changes

• Change POT version to 8.2dev
• Add minigalleries for all part of the quik start guide instead of hand written itemizes in hints that need updating.

Motivation and context / Related issue

Better doc and differentiation of dev verison from stable

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 #334 (266298d) into master (be0ed1d) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #334      +/-   ##
==========================================
- Coverage   93.07%   93.03%   -0.04%     
==========================================
  Files          21       21              
  Lines        5198     5198              
==========================================
- Hits         4838     4836       -2     
- Misses        360      362       +2