Document and make use of the {{{next}}} mustache chunk about pillar

[DamienCassou]

In the mustache template, it is possible to write {{{next}}} to get a link to the next input file in the inputFiles parameter. This is useful to let users read a chapter and click on the link at the end to go to the next. This is already implemented and should work.

What can be done is to make this feature more visible by actually using it in default templates and documenting it.