"Merge requests" is missing `approvals.get()` and etc examples

## Description of the problem, including code/CLI snippet

https://python-gitlab.readthedocs.io/en/stable/gl_objects/merge_requests.html

Gitlab page (https://docs.gitlab.com/api/merge_requests) is of course accurate - but examples always beat `ipython` sessions 😊 

## Expected Behavior


## Actual Behavior


## Specifications

  - python-gitlab version:
  - Gitlab server version (or gitlab.com):
