Add Doxygen build and fix all warnings

[pentschev]

The C++ code was documented for some time but Doxygen build process was not included. This change now introduces Doxygen builds and fixes all documentation warnings.

[pentschev]

@charlesbluca @wence- I'd appreciate your review here sometime next week. Sorry that it is such a long PR, fortunately ~2.8k lines are just the Doxyfile, whereas the remaining is mostly missing documentation.

[charlesbluca]

Thanks @pentschev! So far I've only looked into the files generating the documentation, will follow up on the added docstrings themselves - would it make sense to add codespell as a pre-commit check in this PR as well? There are a few typos that might be easier to review and correct by adding that check and running through the changed files.

[pentschev]

would it make sense to add codespell as a pre-commit check in this PR as well? There are a few typos that might be easier to review and correct by adding that check and running through the changed files.

This is a great idea, I didn't know codespell but was wondering if something like that existed. I've now added codespell and fixed typos it identified.

[pentschev]

Reasoning:

• cancelation: codespell corrects it to the British English spelling "cancellation" (with two ls), whereas we chose the American spelling "cancelation(with onel`) as it is the one used by UCX;
• inflight: that's how we've been writing it and there are many instances of the word "inflight" online, although dictionaries seem to point "in-flight" as correct codespell didn't find all its instances automatically so for now I ignored it, but we can review it if the consensus is we should definitely switch to "in-flight".

[pentschev]

Thanks again @charlesbluca , I've addressed the points you raised. 🙂

[charlesbluca]

Thanks @pentschev!

[pentschev]

Thanks for the review @charlesbluca !

[pentschev]

/merge