2029 update contributing docs.md for references thumbnails viewing rtd build

[bdg221]

Fixes #2029

This PR addresses 3 documentation issues:

1.) Add a section to the documentation for adding references in the Mitiq bibliography when updating / adding documentation.

2.) Add a section to the Jupyter notebook example documentation for adding a thumbnail.

3.) Add a section to the Contributing to the Docs for viewing the Read The Docs from a PR build.

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License

• [ X] I license this contribution under the terms of the GNU GPL, version 3 and grant Unitary Fund the right to provide additional permissions as described in section 7 of the GNU GPL, version 3.

Before opening the PR, please ensure you have completed the following where appropriate.

• [ X] I added unit tests for new code.
• [ X] I used type hints in function signatures.
• [ X] I used Google-style docstrings for functions.
• [X ] I updated the documentation where relevant.
• [X ] Added myself / the copyright holder to the AUTHORS file

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (68b2b12) 98.30% compared to head (028fe24) 98.30%.
Report is 18 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2053   +/-   ##
=======================================
  Coverage   98.30%   98.30%           
=======================================
  Files          87       87           
  Lines        4118     4125    +7     
=======================================
+ Hits         4048     4055    +7     
  Misses         70       70           

see 1 file with indirect coverage changes

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[nathanshammah]

Nice PR, thanks @bdg221!

[andreamari]

Very good and useful! Thanks @bdg221