readthedocs: Use dirhtml builder.

We used this builder before, but from the project configuration
on the website.  ReadTheDocs doesn't allow to change it there
anymore and it doesn't allow to see the full name of the previously
used builder (!!), so I failed to migrate it to the config file.

The result is that older link like:
  https://docs.openvswitch.org/en/latest/howto/dpdk/
Now require .html:
  https://docs.openvswitch.org/en/latest/howto/dpdk.html

Fixing now by switching the builder back.

Fixes: e388bd7 ("readthedocs: Add the configuration file.")
Reported-by: Antonin Bas <abas@vmware.com>
Reported-by: David Marchand <david.marchand@redhat.com>
Reported-at: openvswitch/ovs-issues#310
Reviewed-by: Antonin Bas <abas@vmware.com>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

.readthedocs.yaml

@@ -14,6 +14,7 @@ build:
 # Build documentation in the "Documentation/" directory with Sphinx.
 sphinx:
   configuration: Documentation/conf.py
+  builder: "dirhtml"
 
 # Build all formats: HTML, PDF, ePub.
 formats: all