test: all public API methods should be documented with Javadoc

[monperrus]

With this new architectural test, the CI will check that all public methods added in a PR are documented.

Note that the check is not yet fully complete, because the documentation debt was too high.
There remains small methods that are undocumented (methods with less than 35 expressions):

method.filterChildren(new TypeFilter<>(CtCodeElement.class)).list().size() > 35

upon acceptance, I will a good first issue to document the remaining methods.