Build docs using Doxygen on Travis CI

[DylanVanAssche]

1. What happens:

   • Docs building manual takes too much effort

2. What was expected:

   • Build docs using Doxygen on Travis CI

3. How to reproduce:

   1. N/A

4. Environment:

   • Sailfish OS version: 2.2.0.29
   • Sailfish OS hardware: Xperia X, Jolla 1