ns2: manuals: rework manuals to match ns2
the current manual still describe the old ns1 code.
common/chapters: extend gb/ns2 chapters
Merge gb-variants.adoc into gb-ns2.adoc
Add a general overview of the internal structure which is also used
by the vty.
Move the configuration into a seperate adoc to allow the PCU manual to skip
the part which doesn't apply for it.
Copy the timer configuration into gb.adoc to allow document newer
timers. All users of the NS2 code will use the timer sub-chapter.