So the same file can be used without adaptions in other projects (for
example Qbs).
Adds installation of the html documentation, which
should not hurt, or might even be preferable.

Change-Id: I8f6fbf072b348dfe13e54a5f2018ed6e24fc3a34
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>