Bring back register_menu decorator

## Is your feature request related to a problem? Please describe.
I have used `register_menu` a lot in my code in lots of different files, but it got very abruptly removed when upgrading flask_menu.

## Describe the solution you'd like
I would like it to stay. If not possible I think the following steps should be taken.
- It should be deprecated for a while and show a warning when used.
- It should be removed from the documentation here: https://flask-menu.readthedocs.io/en/latest/
- An example should be added to the documentation that describing alternative ways to accomplish the same.
- It should be added to the changelog in the documentation above

## Describe alternatives you've considered
- Staying on version <2.0.0. (But then I'm stuck with an old version of flask)
- Reimplement it myself and monkey-patching this module (if possible)

## Additional context
I don't know why this was removed.

Start by checking the current public API and the documentation at flask-menu.readthedocs.io to determine how register_menu was removed and what alternatives exist. The issue does not name source files or tests. Done would require either restoring register_menu with a deprecation path or documenting an agreed alternative, plus updating the documentation and changelog.

Written by the indexing model from the issue text.