Configurable next links

[jroper]

Paradox currently supports the ability to render next links, which I presume is calculated from the structure extracted from the index containers.

We have a new requirement on Lagom to be able to control the next links, for example, we support both Maven and sbt for building, and we want the next link after the page that describes the prerequisites to actually be two links, one to the Maven page, one to the sbt page, and then, if Maven comes first in the index, it's next page should not be the sbt page, but the page after that.

There's a few ways this could be provided, we could create a new directive, eg @navref, that allows specifying this:

@navref:[Prerequisites](prerequisites.md) { next=sbt.md,maven.md }

Alternatively we could perhaps hijack the title text, allowing the inclusion of metadata in there, eg:

[Prerequisites](prerequisites.md ";next=sbt.md,maven.md")

In that case, the end of the title text is delineated by ;.

[jonas]

I suggest to configure the "next page" in the front matter of the Maven and sbt pages. This would allow to either implement it entirely in the theme or change Paradox to read it.