Allow multiple Mount objects with the same prefix (fixes #2374)

[nicolaipre]

Summary

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

This is a proposed fix to solve #2374, similar to the proposed fix for Host() in #2263.

This allows users to have multiple Mount() objects with the same path/prefix set, but use a different name. In essence it allows users to have named route groups, allowing the Mount names to be used directly in the OpenAPI spec, etc...

[nicolaipre]

After thinking this through with a clear mind it actually isn't necessary at all. I got so stuck in trying to figure out the other routing issues related to Host that I completely lost track of what I was actually trying to solve here.

It is good that it is the way it is now, because if you notice a route not resolving after putting it in a Mount() it is most likely that you have a duplicate Mount, which will let you know and keep your code clean.

Closing the PR and related discussion.