Update readme to properly credit Grant Sanderson and revise dismissive tone

[czuzu]

Overview: What does this pull request change?

The current repository description contains a phrase that sounds dismissive of Grant Sanderson’s involvement in what concerns the Manim tool. I'm not trying to downplay the community's role, instead I find it important to give proper credit to the original author while also acknowledging community improvements over his original work. The line "not associated with Grant Sanderson or 3Blue1Brown in any way" downplays his role as the original creator and key contributor to the tool. I'm pretty sure I'm not the only one feeling like that when reading that phrase, especially people that have watched and enjoyed his videos and know he is also the original author of the tool.

The CE home-page documentation here: https://docs.manim.community/en/stable/ also omits mentioning anything about him or his YouTube channel, which is unfair.

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

Proper credit to the original author.

Further Information and Comments

Please also update the home-page documentation: https://docs.manim.community/en/stable/

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