Reorganize the help system

[pcav]

The idea is to unify the manual and the help, so the user can go to the appropriate documentation page directly from QGIS interface.
Writing docs for Processing algs would be also easier, as it can be done in rst (or markdown) and displayed in Processing help, as it happens now for e.g. GRASS manpages.

[rduivenvoorde]

We have this idea about a system online (or possibly via a per language downloaded zip), see

qgis/QGIS-Enhancement-Proposals#51

I still think this is a good idea

[pcav]

Agreed. IMHO we should really make it for Q3.

[alexbruy]

PR which implements qgis/QGIS-Enhancement-Proposals#51 merged in master. So we can start with adopting docs, merging context help and linking them from QGIS

[m-kuhn]

Thanks @alexbruy for the excellent work!

[nyalldawson]

This was done for 3.0