doc: fix sphinx rtd theme import in GHA

doc: fix sphinx rtd theme import in GHA

If the rtd theme is available, passing it by name is enough to select
it. Sphinx itself recognises the "sphinx_rtd_theme" name as a special
case and tries to find its path automatically.

On the other hand, passing a html_theme_path makes sphinx parse all
themes availables in this path, which in some environment (like GHA) is
/usr/share and makes sphinx error on the first zipfile it finds (in GHA,
some Azure CLI thingy) that has no sphinx theme in it.

Fixes: 46562be65094 ("doc: import sphinx rtd theme when available")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Aaron Conole <aconole@redhat.com>