Publish C++ developer docs

[vyasr]

Description

This PR adds the developer documentation to our doxygen. The changes are minimal: the files have been moved from cpp/docs to cpp/doxygen and added to the INPUT section of the Doxyfile, and a custom DoxygenLayout.xml has been added to include the necessary header (note that this file was autogenerated with doxygen -l and the only modification was adding one tab for the developer guide). I added an anchor to the main developer guide header so that it can be linked to from the header.

This PR is currently a draft because our developer documentation does not all render well yet inside doxygen. The issues that I notice so far:

1. Code font does not render in headers, so all the headers that are classes or functions need backticks removed.
2. All @ symbols will by default be interpreted by doxygen, so examples of documentation inside our doxygen need to have those escaped.
3. The doxygen pre-commit check will also interpret parts of these files, so we essentially need to make sure that all doxygen there is valid. Fixing this is honestly probably worthwhile anyway.

Assuming nobody views changing these as dealbreakers, I think we can safely move forward here and I can make the necessary changes. This is how it looks now:

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (branch-22.10@ecf4662). Click here to learn what that means. Patch has no changes to coverable lines.

Additional details and impacted files

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[vyasr]

@gpucibot merge

[vyasr]

rerun tests