doc: make sure docs have been generated when installing

doc: make sure docs have been generated when installing

This is done in three steps:

 - first a "custom install target" is created for each custom target
   which builds documentation;

 - then dependencies between those custom install targets and the
   correspondent "build" targets are added;

  - finally some code is added to ensure those "custom install targets"
    are called when the builtin install target is executed.