build: non-recursive doc + parallel sphinx

build: non-recursive doc + parallel sphinx

Sphinx actually does work with a parallel build, if the doctree creation
is a separate step (which the other builds will then just read
unmodified.)  This can be done with the "dummy" target.

This also adds "-j6" to sphinx-build and adds a "--disable-doc-html"
switch on ./configure to turn on/off building HTML docs separately.

Also, HTML docs are now installed by "make install" to
/usr/share/doc/frr/html.

Signed-off-by: David Lamparter <equinox@diac24.net>