docs: use parallel builds to speedup docs generation

Using parallel builds to increase doc regeneration as well as speedup
Travis builds.

With hot filesystem speed are as follows depending on the -j value:

Processes	Duration
1	7.75s
3	5.2s
5	4.21s
8	3.88s
10	3.96s
15	3.38s
20	3.48s

Cold (12 runs)
1	20.27 (17.82-22.71)
15	17.91 (14.41-21.42)

Not as big a saving as I'd hoped for a cold start but good for doc
development.