⬆️ Bump nbconvert from 6.3.0 to 6.5.1 in /docs

Bumps [nbconvert](https://github.com/jupyter/nbconvert) from 6.3.0 to 6.5.1.
- [Release notes](https://github.com/jupyter/nbconvert/releases)
- [Commits](jupyter/nbconvert@6.3.0...6.5.1)

---
updated-dependencies:
- dependency-name: nbconvert
  dependency-type: indirect
...

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