Update CONTRIBUTING.md and add documentation improvements hints

[bdovaz]

In the CONTIBUTING.md file right now it does not record any information as to how to test and generate the documentation.

This PR helps to make the documentation better.

This post is the one that helped to improve it #2220 (comment)

[codecov-commenter]

Codecov Report

Merging #2228 (31f2aaa) into main (964fe7f) will increase coverage by 0.51%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2228      +/-   ##
==========================================
+ Coverage   82.47%   82.99%   +0.51%     
==========================================
  Files         170      170              
  Lines        4469     4470       +1     
==========================================
+ Hits         3686     3710      +24     
+ Misses        783      760      -23     

Impacted Files	Coverage Δ
megalinter/utils.py	82.96% <0.00%> (ø)
megalinter/linters/DotnetFormatLinter.py	100.00% <0.00%> (ø)
megalinter/Linter.py	84.69% <0.00%> (+0.02%)	⬆️
megalinter/reporters/UpdatedSourcesReporter.py	89.74% <0.00%> (+2.56%)	⬆️
...alinter/tests/test_megalinter/helpers/utilstest.py	89.01% <0.00%> (+8.05%)	⬆️

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[nvuillam]

Great :)