Merge pull request #14700: olinks for NixOS manual

This allows to use <olink> tags inside NixOS options to reference
sections from the manual. I've originally introduced it in #14476 to
reference the Taskserver specific documentation from the options
reference but as suggested by @nbp, this was done as a separate pull
request to ensure greater visibility rather than being "hidden" in the
Taskserver branch.

The build time for the manual is around 30s on my machine without this
change and 34s with this change, so it shouldn't have a very big impact
on the build time of the manual.

Olinks between the options reference and the manual now will look like
this:

"More instructions about NixOS in conjuction with Taskserver can be
found in the NixOS manual at Chapter 15, Taskserver."

More documentation about olinks can be found here:

http://www.sagehill.net/docbookxsl/Olinking.html



Acked-by: Eelco Dolstra <eelco.dolstra@logicblox.com>