Manim directive duplicates code

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## Description of bug / unexpected behavior
When using manim_directive for including Manim videos in a Sphinx document, the code snippet is duplicated in the rendered output.
For example, given this .rst file:

```rst
My Plugin Documentation
=======================

Test
----

TODO

.. manim:: MyScene

class MyScene(Scene):
def construct(self):
circle = Circle()
self.play(Create(circle))
self.wait()
```

This is the result:


## Expected behavior
The second code snippet should not be there. A workaround is removing the relative HTML code, that is:

```HTML

class MyScene(Scene):

    def construct(self):

        circle = Circle()

        self.play(Create(circle))

        self.wait()

```

This is the result:


## How to reproduce the issue
1. Use manim_directive with a simple scene, as shown in the example below.
2. Render the scene.

## System specifications

System Details

- OS: macOS 12.7.5 (Monterey)
- RAM: 8GB
- Python version (`python/py/python3 --version`): 3.12.6

Start by reproducing the issue with the provided .rst example and inspect the manim_directive output during Sphinx rendering. Compare the generated HTML with the workaround, and consider the issue done when the rendered scene contains the video without a duplicated code snippet.

Written by the indexing model from the issue text.