Fix installation of HTML manual

Simplify things by making the source distribution and the git repository
have the same layout (it wasn't working earlier at all, and just adds
complexity if I were to try to fix it).  This allows using the same
installation rules for both (just replacing the binary directory with
the source directory).  Install the output directory under the proper
name (html/ instead of final/).

Change-Id: Ica88a0825999f201e9856e11e1784bd4acc20c91