[Prototype]: Add button to open demos in Xanadu Cloud

[trbromley]

This is a prototype PR to add a button at the top of each demo that links to the corresponding notebook version of the demo in Xanadu Cloud.

There is no need to review at this stage.

[github-actions[bot]]

Thank you for opening this pull request.

You can find the built site at this link.

Deployment Info:

• Pull Request ID: 536
• Deployment SHA: 17c898a17d3d47ed2d5e02d3ff3a3f37c4098721 (The Deployment SHA refers to the latest commit hash the docs were built from)

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[trbromley]

Closing this PR because the build is no longer working.