docs: build an index page for the HTML docs

docs: build an index page for the HTML docs

There is no index.html start page for the QEMU HTML documentation.  An
index page is needed so that documentation can be browsed easily on the
web.

This patch adds an index.html.in template file where the QEMU version
number is expanded.  It is written in HTML instead of using the existing
sphinx (rST) and texi documentation generators because they are
heavyweight and would make this harder.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20191219135620.1626608-1-stefanha@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>