Generate code coverage reports for C++ code

[Krinkle]

It looks like this may've worked in the past: ​http://docs.wildfiregames.com/ -> ​http://docs.wildfiregames.com/coverage (404 Not Found)

Getting it going again would potentially allow new contribtors like myself to find small corners to start contributing to. For example, a component that might have some test coverage already where I might add an additional case, learn a bit about how tests work and the code under test.

Also probably not too bad to have in general as a reference to browse from a URL to see how well-tested various areas are without having to find out locally in the code.

[Stan]

I have managed to generate the C++ coverage locally. Will work in the coming days to put it on the website. Here is the generated code. ​https://upload.jabberfr.org/nk6ucbCZ5V4k-ocE/coverage.zip

[Stan]

In 24520:

Add a pipeline to generate test coverage.

Fixes: #5739