Added documentation for lagged animations

[alembcke]

Overview: What does this pull request change?

Added documentation for lagged animations, including AnimationGroup, Succession, LaggedStart and LaggedStartMap.

Motivation and Explanation: Why and how do your changes improve the library?

There was no documentation for lagged animations and several questions in Discord on the subject.

Links to added or changed documentation pages

https://manimce--2953.org.readthedocs.build/en/2915/reference/manim.animation.composition.html

Further Information and Comments

N/A

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