Added thematic guide for the manim-voiceover plugin

[osolmaz]

Overview: What does this pull request change?

This adds a small section in the docs about using Manim Voiceover to add voiceovers to videos.

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

Docs are the main resource for learning Manim. It would be nice if people get to know about Manim Voiceover while using it, because it solves a major pain point while developing Manim animations

Links to added or changed documentation pages

https://manimce--3049.org.readthedocs.build/en/3049/guides/add_voiceovers.html

Further Information and Comments

I added a new page under Thematic Guides. Feel free to propose something else.

Also, I refrained from including a video and just linked the examples gallery https://voiceover.manim.community/en/latest/examples.html
Getting an example video included in the build is not viable, because it both requires committing mp3 files to the repo and installing a plugin in the build container.
If you think a video would be nice, I could link one of the GitHub hosted videos like I do in Manim Voiceover docs.

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

[behackl]

This is a nice addition, thank you! You could also have written the tutorial as a markdown file, but now that it is already ReST (and is formatted correctly, as far as I can tell), it doesn't make sense to change it either.

Perhaps anyone who has used manim-voiceover before a bit more wants to chime in and comment on this PR, but from my point of view it is fine to include the tutorial like this.

I've left one (non-blocking) comment, please take a look.