Add glossary section to documentation

## Enhancement proposal

Would be really helpful to have a Glossary section for key terms to do with Manim to make it easier to get up and running without getting lost in terms which may not be clear

## Additional comments

Similar items have been done for [Python](https://docs.python.org/3/glossary.html) and [Numpy](https://numpy.org/doc/stable/glossary.html)

Start by reviewing Manim's existing documentation structure and the Python and NumPy glossary examples linked in the issue. Done means a glossary section is added to the documentation with clear definitions for key Manim terms and is linked where readers can find it.

Written by the indexing model from the issue text.