docs: attempt to fix .readthedocs.yaml

Seems like we need to update the config ([0],[1]).

  [0] https://blog.readthedocs.com/migrate-configuration-v2/
  [1] https://blog.readthedocs.com/use-build-os-config/

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

docs/conf.py

@@ -18,6 +18,7 @@ extensions = [
     'sphinx.ext.viewcode',
     'sphinx.ext.imgmath',
     'sphinx.ext.todo',
+    'sphinx_rtd_theme',
     'breathe',
 ]
 

docs/sphinx/requirements.txt

@@ -1 +1,2 @@
-breathe
+breathe
+sphinx_rtd_theme