Add Doxyfile

[TobiasFella]

Next steps would be:

• Adding a CI job to build the documentation
• Hosting it somewhere

Where to host it?
I could put it on my server, or we can use github pages

It's almost completely the default template, since I don't really know anything about doxygen - seems to mostly work though

@CarlSchwan do you know anything useful about how doxygen should be configured (any maybe make it less ugly? 😄)

Current status can be viewed at quotient.tobiasfella.de

[TobiasFella]

Some problems I noticed:

• on pages like https://quotient.tobiasfella.de/classRoom.html , it says Definition at line 82 of file room.cpp - which is very wrong, the class is defined in the header; the line it points to is the definition of the private class
• In https://quotient.tobiasfella.de/annotated.html, some classes are shown on the top-level of the tree, while others are below the Quotient namespace

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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[TobiasFella]

Some problems I noticed:

* on pages like https://quotient.tobiasfella.de/classRoom.html , it says `Definition at line 82 of file room.cpp` - which is very wrong, the class is defined in the header; the line it points to is the definition of the private class

* In https://quotient.tobiasfella.de/annotated.html, some classes are shown on the top-level of the tree, while others are below the `Quotient` namespace

Fixed by not building docs for .cpp files

[KitsuneRal]

I have no prior experience with Doxyfiles but my uneducated suggestions are below :) Also: THANKS!

[KitsuneRal]

One more thing I forgot: I wonder if using the Clang parser would help with recognising [[deprecated]] declarations that don't have \deprecated counterparts in doc-comments.

[TobiasFella]

I'm also proposing to use the theme currently deployed at https://quotient.tobiasfella.de/ (not in the merge request yet); opinions?

[KitsuneRal]

Thanks a lot! A nitpick about the shell override.