[Feature Request]: Cross-repo snippets/includes

### Prerequisites

- [x] I have searched existing issues to ensure this feature hasn't already been requested
- [x] I have tested using the latest version of docs-builder

### What problem are you trying to solve?

Snippets and more generally reuse in documentation are a great way to display the important information anywhere it's needed while maintaining a single source of truth.

It would be super awesome if documentation files in different repositories could also share snippets.

This actually is not a simple nice to have, it's rather important in a multi-repo docs setup like ours

### Proposed Solution

Just like we do with cross-repo links, what if we could include files with similar links, like `{include} repo://path_to_snippet`

### Examples and Research

_No response_

### Alternative Solutions

_No response_

### Additional Context

_No response_

### How important is this feature to you?

Important