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 didn't 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.

Signed-off-by: Ilya Maximets <[email protected]>

.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