Create llms.txt

[sanjeed5]

Overview: What does this pull request change?

Add's a llms.txt file for Manim. Reference: https://llmstxt.org/

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

I was trying to use an agent to work with Manim and thought of this

Further Information and Comments

I don't know if this is the right position for the file to be in, please comment and I"ll make necessary changes to complete this.

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

[JasonGrace2282]

I feel like this would be something better and easier to keep up to date for Sphinx (see sphinx-doc/sphinx#13268), but they seem to not have any interest in it currently.