docs: Update agent development and integration testing documentation

[nr-ahemsath]

I went over all three documents in docs and updated them to the current state of the world.

Also, as part of this process, I fixed an issue with the Linux profiler build docker-compose file where it would bomb out if a certain env var wasn't set, even though that env var is only required for building a debug version of the Linux profiler, which is not relevant to any ongoing development or testing processes. The fix is to fall back to a default value if the env var isn't set.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.22%. Comparing base (6737414) to head (c37b6f4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3063      +/-   ##
==========================================
- Coverage   82.22%   82.22%   -0.01%     
==========================================
  Files         477      477              
  Lines       30627    30627              
  Branches     3426     3426              
==========================================
- Hits        25184    25183       -1     
- Misses       4643     4648       +5     
+ Partials      800      796       -4     

Flag	Coverage Δ
Agent	83.28% <ø> (-0.01%)	⬇️
Profiler	72.51% <ø> (ø)

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see 4 files with indirect coverage changes

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