chore(docs): document HTML custom templates (#357)

* chore(docs): document HTML custom templates

Shows an example of custom template.

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

Closes #356

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* chore(docs): document changes and fix

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

docs/source/contributing/workflow.md

@@ -4,7 +4,7 @@ This document is there to help you recreate a working environment for Manim Slid
 
 ## Dependencies
 
-```{include} ../quickstart.md
+```{include} ../installation.md
 :start-after: <!-- start deps -->
 :end-before: <!-- end deps -->
 ```