Prepare notebooks for central examples

[Yoshanuikabundi]

We're moving towards a central examples page for OpenFF!

• Preview here: https://openff-docs--7.org.readthedocs.build/en/7/examples.html
• Rendered + executed notebook example: https://openff-docs--7.org.readthedocs.build/en/7/examples/openforcefield/openff-nagl/train-gnn-notebook/train-gnn-notebook.html
• Draft PR here: Examples page openff-docs#7

This PR just updates the examples to make them work for that purpose. Dependency install instructions are no longer necessary in each notebook because they're being unified in the central examples downloads (see the top of each notebook, though the CSS styling is not final). This should make it easier to both find and consume examples.

Changes to examples are synced when they make their way to conda forge - so users will always have examples that are consistent with the released version. Only the latest version is rendered, but users will be able to find earlier versions through the "View on GitHub" link. Links to run the notebook in Colab or download it with a helpful dependency installation script are also provided.

Changes made in this Pull Request:

• Changes titles to make them clear outside the context of NAGL
• Removes dependency installation instructions, as that is being unified with other examples

PR Checklist

• CHANGELOG updated?

[lilyminium]

This is super cool, thanks Josh!