🩹 add graphviz to the RtD dependencies (#340)

## Description

This PR adds graphviz to the RtD dependencies so that the documentation
renders properly.

## Checklist:

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your pull request will be merged swiftly.
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- [x] The pull request only contains commits that are related to it.
- [x] I have added appropriate tests and documentation.
- [x] I have made sure that all CI jobs on GitHub pass.
- [x] The pull request introduces no new warnings and follows the
project's style guidelines.

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

.readthedocs.yaml

@@ -8,6 +8,8 @@ build:
   os: ubuntu-22.04
   tools:
     python: "3.11"
+  apt_packages:
+    - graphviz
   jobs:
     post_checkout:
       # Skip docs build if the commit message contains "skip ci"