[WIP] Added docs for MovingCamera, MultiCamera and (soon) ImageMobjectFromCamera

[chopan050]

Overview: What does this pull request change?

As requested by @MrDiver, I'm currently working on adding docs for MultiCamera (which inherits from MovingCamera) and soon ImageMobjectByCamera as well.

These classes seem to have the potential to be tremendously useful, but they're currently very undocumented and seemingly not really used by anyone.

THIS IS A WORK IN PROGRESS.

Links to added or changed documentation pages

• MovingCamera
• MultiCamera

Reviewer Checklist

• [ ] The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• [ ] If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• [ ] If applicable: newly added functions and classes are tested