chore(docs): document HTML custom templates

[jeertmans]

Shows an example of custom template.

TODO:

• [x] finish documentating;
• [x] add possibility to pass -cargs to Manim Slides' convert method when calling the Sphinx extension.

Closes #356

[codecov[bot]]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (02a8173) 80.60% compared to head (137a358) 80.33%.

Files	Patch %	Lines
manim_slides/docs/manim_slides_directive.py	0.00%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #357 +/- ## ========================================== - Coverage 80.60% 80.33% -0.27% ========================================== Files 21 21 Lines 1789 1795 +6 ========================================== Hits 1442 1442 - Misses 347 353 +6 ```

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