Convert C++ documentation to Doxygen format.

[ChrisCummins]

Current progress:

• compiler_gym/envs/llvm/service
• compiler_gym/service
• compiler_gym/service/runtime
• compiler_gym/util

[codecov-commenter]

Codecov Report

Merging #286 (de14bfa) into development (bd9bff9) will increase coverage by 0.02%.
The diff coverage is 90.00%.

@@               Coverage Diff               @@
##           development     #286      +/-   ##
===============================================
+ Coverage        82.42%   82.45%   +0.02%     
===============================================
  Files               87       87              
  Lines             4729     4748      +19     
===============================================
+ Hits              3898     3915      +17     
- Misses             831      833       +2     

Impacted Files	Coverage Δ
compiler_gym/service/compilation_session.py	64.70% <ø> (ø)
compiler_gym/spaces/named_discrete.py	94.73% <ø> (ø)
compiler_gym/wrappers/core.py	88.57% <88.23%> (-0.32%)	⬇️
...ice/runtime/create_and_run_compiler_gym_service.py	100.00% <100.00%> (ø)
compiler_gym/wrappers/__init__.py	100.00% <100.00%> (ø)

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