Adds "Previous" and "Next" links in the documentation

[tdziurko]

While reading Undertow docs I've missed links allowing me to go to the next section while reading whole documentation. And this PR attempts to fix it.

Screenshow with example: http://screencast.com/t/P0RbP8bkHi

[stuartwdouglas]

The problem with this is that it is really fragile, and easy to break if docs are re-ordered, renamed etc.

I might have a look at something to make this happen automatically.

[tdziurko]

Yes, I agree. I've tried to google for something automatic but this is my first asciidoc experience. If you find something better I could to the boring part :)

[torakiki]

I'm going through the docs now and I too feel that a previous and next a link are needed.

[tdziurko]

:) But there are two ways: either we could have no Prev / Next links or have them but fragile for any refactoring.