Avoid documenting private or external classes

There are a lot of monkey patches inside the code base but there's
no need to document external constants so let's remove them from
the documentation

Also, since there are monkey patches for some test cases classes,
there were sometimes both documented and sneaked under the wrong
section in the sidebar.

Finally, for future references, the `active_support/vendor`
folder has been originally ignored in https://git.io/vDqfA but
no longer exists.

[ci skip]

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