support the the `":include"` helper, or similar, for _sidebar and _navbar.

## Feature request

#### What problem does this feature solve?

To include an external page, one must create a new markdown file with the `[include](//foo.com/foo ':include')` inside of it, then they can link to the local file from the sidebar. That's a lot of work.

#### What does the proposed API look like?

An update would allow us to include directly from sidebar or navbar, like so:

```markdown

- Menu:
- [Page from outside](https://outside.com/foo.md ":include")
```

This would cause the sidebar link to load the content from the outside location, and we don't need to make a wrapper file.

#### How should this be implemented in your opinion?

Need to update the sidebar and navbar compilers to support features like main compiler. The code is already duplicated twice between client renderer and server renderer, and we don't want more duplicates, so we would need to consolidate code and re-use it well.

#### Are you willing to work on this yourself?