Add functionality in the include directive to link to remote files (in addition to local files)

[firasm]

Context

I use JupyterBooks for my course website, often because I use Jupyter Notebooks and thebe, RiSE, etc for my CS courses.

I use the include directive to link to local md files as part of my unsyllabus like this:

```{include} unsyllabus_bits/course_tools.md
```

Proposal

This term I am teaching three different courses and have three different JupyterBooks, so rather than edit these files separately each time I want to make a change, I'd like to remove this redundancy by linking to an .md file that I have remotely in another repository:

```{include} https://raw.githubusercontent.com/ubco-cmps/cosc122_course/main/about/unsyllabus_bits/course_tools.md
```

The reason I can't do this locally (for e.g., outside of the existing repo) is because I rely on GitHub Actions to build my jupyter books automatically and publish them to a remote server.

I'm willing to help contribute to the codebase here (assuming I have the necessary technical expertise), but I'd love for someone to point me in the right direction of how to enhance this functionality. For example, one idea I had was to build a custom directive, something like {include_remote}, but not sure if that's the right way to go.

Tasks and updates

No response