Include classes in Legacy in Doxygen documentation

Moritz Kiehnrequested to merge
470-classes-in-legacy-aren-t-included-in-doxygen-documentation-was-acts-469 into master

Update doxygen config and remove the build badges from the README. They are now listed as project badges directly on Gitlab.

Closes #470 (closed).

Edited by Paul Gessinger-Befurt

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