🚸 allow using raw RST cells in jupyter notebooks (#433)

## Description

This PR updates the jupyter notebook cleaning pre-commit check to
preserve MIME metadata. This allows to use raw RST cells within jupyter
notebooks, which can be very helpful for using Sphinx references as part
of the documentation.

## Checklist:

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- [x] The pull request only contains commits that are related to it.
- [x] I have added appropriate tests and documentation.
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- [x] The pull request introduces no new warnings and follows the
project's style guidelines.

Signed-off-by: burgholzer <burgholzer@me.com>

.pre-commit-config.yaml

@@ -33,6 +33,11 @@ repos:
     rev: 3.2.0
     hooks:
       - id: nb-clean
+        args:
+          - --remove-empty-cells
+          - --preserve-cell-metadata
+          - raw_mimetype
+          - --
 
   # Handling unwanted unicode characters
   - repo: https://github.com/sirosen/texthooks