docs: update readthedocs.org config

RTD is removing the option to use pre-installed packages.  Instead, we
just need to tell them where our Python requirements.txt file is
located so that they'll install exactly those packages.

Signed-off-by: Jeff Squyres <jeff@squyres.com>

Author: jsquyres

.readthedocs.yaml

@@ -13,6 +13,10 @@ build:
   tools:
     python: "3.10"
 
+python:
+  install:
+    - requirements: docs/requirements.txt
+
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: docs/conf.py