Allow snippets to reference files from project root

For project documentation with deeply nested pages it would be helpful to support referencing files using absolute paths which resolve against for example the project root.

An example from Akka HTTP docs:

file: /docs/src/main/paradox/scala/http/common/http-model.md

## Instead of:
@@snip [ModeledCustomHeaderSpec.scala](../../../../../../../akka-http-tests/src/test/scala/akka/http/scaladsl/server/ModeledCustomHeaderSpec.scala) { 

## Use the following:
@@snip [ModeledCustomHeaderSpec.scala](/akka-http-tests/src/test/scala/akka/http/scaladsl/server/ModeledCustomHeaderSpec.scala) { 

The root could be configurable via the SBT plugin. The main downside is that links won’t be “GitHub friendly”.

The alternative is to not nest documentation files inside src/main/paradox but instead in for example /docs/ and example source code somewhere other than /docs/src/main/scala.

[bantonsson]

We use the GitHub friendly feature in our internal docs that also use paradox, not to mention that I can effortlessly jump from the markdown doc to the snippet doc in my editor (yes emacs does that).

If you think the nesting is too deep, then just redefine the sbt setting sourceDirectory in paradox := "/path/to/nirvana".

[jonas]

I was mainly thinking of the sanity of developers maintaining the Akka HTTP documentation, and completely understand the hesitation to break editors and other tools.

Personally, I like the idea of documentation coexisting with each project. So another possibility would be to see if Paradox files can somehow be moved into each subproject thus make linking easier. This way examples would also naturally be part of the subproject’s test code.