docs: specify sphinx dirhtml builder

With the requirement of a configuration file, the default builder
of `dirhtml` that RTD used to use is no longer specified.
This leads to URLs ending in `.html` now, which
breaks other exisitng references.

Refs: psf#6603
Refs: https://docs.readthedocs.io/en/stable/config-file/v2.html#sphinx-builder
Refs: https://www.sphinx-doc.org/en/master/usage/builders/index.html#sphinx.builders.dirhtml.DirectoryHTMLBuilder

Signed-off-by: Mike Fiedler <miketheman@gmail.com>

.readthedocs.yaml

@@ -13,6 +13,7 @@ build:
 # Build documentation in the "docs/" directory with Sphinx
 sphinx:
   configuration: docs/conf.py
+  builder: "dirhtml"
 
 # Optionally build your docs in additional formats such as PDF and ePub
 formats: