docs: use parrallel builds to speedup docs generation

Using parrallel 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)

Processes | Duration
---------------|----------
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.