docs: contributing how to build

[jibel]

Expanded the TODOs to proper build documentation plus some other minor fixes.

Instructions have been tested on a clean 24.04 system.

UDENG-3430

[3v1n0]

So, a part few nits, the PAM side is incomplete since we rely also on the executable that is what actually does the job.

Not sure if we should also mention how service files should be generated / copied, but it's not strictly about building.

[didrocks]

I have 2 small requests to ensure we don’t vendor and stuck Rust dependencies forever and that we install the correct dependencies for code generation.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.46%. Comparing base (d000437) to head (604bab5).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #412      +/-   ##
==========================================
+ Coverage   84.38%   84.46%   +0.07%     
==========================================
  Files          77       77              
  Lines        6691     6693       +2     
  Branches       75       75              
==========================================
+ Hits         5646     5653       +7     
+ Misses        731      730       -1     
+ Partials      314      310       -4     

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[didrocks]

Thanks for the changes!