Add custom admonition for referencing other doc pages

[jacobtomlinson]

Added a custom docref admonition to help cross-reference docs when writing.

You can link to another documentation page with a docref admonition.

```{docref} /cloud/gcp/gke
For more detailed information on launching GPU powered Kubernetes clusters on Google Cloud see the documentation.
```

Renders as:

Note
The Visit the documentation >> link is added automatically in the bottom right based on the page that is referenced in the directive argument.

[jacobtomlinson]

I'm going to merge this because I want to use it in other PRs. If folks have review feedback we can iterate in follow-up PRs.