fdo#86738 add links.txt references to next and previous buttons

Change-Id: I04514498e9f6466416d08eb0e1736dcebe931e71
Reviewed-on: https://gerrit.libreoffice.org/13186
Reviewed-by: Yousuf Philips <philipz85@hotmail.com>
Tested-by: Yousuf Philips <philipz85@hotmail.com>