[AIRFLOW-XXXX] Google Season of Docs updates to CONTRIBUTING doc

[potiuk]

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Jira

• In case you are fixing a typo in the documentation you can prepend your commit with [AIRFLOW-XXX], code changes always need a Jira issue.

Description

• Here are some details about my PR, including screenshots of any UI changes:

This is first part of improvements coming from @ElenaFedotova as part of Google Season of Docs initiative.

This is quite a big change in structure and content of the documentation (including format change). You can see the diff between the old and new doc content here:

https://www.diffchecker.com/LWDaX6bW

I hope this is a nice step to improve our "contributing" documentation.

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.

[codecov-io]

Codecov Report

Merging #6283 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6283      +/-   ##
==========================================
- Coverage   80.34%   80.33%   -0.02%     
==========================================
  Files         616      616              
  Lines       35738    35738              
==========================================
- Hits        28714    28709       -5     
- Misses       7024     7029       +5

Impacted Files	Coverage Δ
airflow/models/taskinstance.py	93.26% <0%> (-0.51%)	⬇️
airflow/utils/dag_processing.py	56.55% <0%> (-0.35%)	⬇️

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[kaxil]

In the first commit msg

- Co-authored-by: Elena Fedotova <lavel@mail.ru>>
+ Co-authored-by: Elena Fedotova <lavel@mail.ru>

[potiuk]

I just rebased this change on top of the latest master and squashed all the changes (leaving co-authors ;).

@ashb @kaxil @mik-laj -> I think this one is quite ready to be merged (I like it a lot more now :). We have few things that we work with @efedotova on #6285 which will further update the CONTRIBUTING.rst. One of the things is to move the "testing" section from Breeze to CONTRIBUTING so I'd love to merge this one and rebase the $6285 on top of it.

One thing to note - we've changed the format/URL of CONTRIBUTING.md to .rst (which is a good thing) but maybe a sensible approach will be to leave CONTRIBUTING.md with link to CONTRIBUTING.rst rather than delete CONTRIBUTING.md ? There are some slack messages and other places pointing to the old CONTRIBUTING.md I am afraid.

What do you think ?
Can you review and approve please :).

[potiuk]

I've added the note to Contribuing.md and updated the links in Airflow's code.

[potiuk]

I also reviewed and updated all the cross-links to different sections of both CONTRIBUTING and BREEZE