docs: use parrallel builds to speedup docs generation

Using parrallel builds to increase doc regeneration as well as Travis
builds.

With hot filesystem speed are as follows:

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

Cold
1	21.9s