[MRG] New Quickstart guide and revamp User guide

[rflamary]

Types of changes

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

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 97.09%. Comparing base (928a67a) to head (e68f54e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #726      +/-   ##
==========================================
- Coverage   97.10%   97.09%   -0.01%     
==========================================
  Files         100      100              
  Lines       20428    20432       +4     
==========================================
+ Hits        19837    19839       +2     
- Misses        591      593       +2     

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