Improve documentation for contributors

[olethanh]

Split of from #719, to improve reviewability.

Document how to do the developer setup.

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 62.56%. Comparing base (4d319f0) to head (9e444d2).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
tests/supervisor/test_instance.py	0.00%	1 Missing and 1 partial ⚠️
src/aleph/vm/models.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #723      +/-   ##
==========================================
- Coverage   62.57%   62.56%   -0.01%     
==========================================
  Files          70       70              
  Lines        6233     6235       +2     
  Branches      503      503              
==========================================
+ Hits         3900     3901       +1     
- Misses       2180     2181       +1     
  Partials      153      153              

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[Psycojoker]

LGTM in general, I've just added a few typos fixes and suggestions.