Provide a default include path, so that it doesn't need to be relative to the current file

[brizental]

This is a minor feature request.

I'd like to be able to define a path where mdbook will look for includes, this way, if I change the folder structure the include paths don't break necessarily.

I would be happy to work on this feature, in case it is decided that it is ok to implement it :)

[daprilik]

At minimum, it would be great if {{ #include /book/root/relative/path }} worked correctly (much like book-root relative markdown links work).

I've worked around this using a custom pre-processor which does this translation manually, but this really seems like something that mdbook should support out-of-the-box.