Make next and previous (global variables) respect toc structure

[mgoulao]

Considering the following toc for index:

.. toctree::
  a
  b

where a has:

.. toctree::
  a1
  a2

and b has:

.. toctree::
  b1
  b2

Currently, page a2 next has a link to b and b has a previous to a2, however, in some cases, this relation doesn't make sense. So instead of having these relations:

a<->a1<->a2<->b<->b1<->b2

I would like a configuration variable that would change this relation to

a<->a1<->a2     b<->b1<->b2

My current workaround is to manually add a meta variable, e.g. :firstpage: or :lastpage:, where I want to remove the next and/or the previous button and extend the theme's template to add that logic. But, it requires me to change the files every time I change the structure or order.