docs: Update link to the php bindings project webpage

docs: Update link to the php bindings project webpage

The page for the libvirt-php project is now hosted via gitlab pages and
available at https://php.libvirt.org/

Additionally drop the docs/php.rst(html) page which has only redundant
information.

Redirects will be set up to make sure old links still work.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>