add docs requirements

.readthedocs.yaml

@@ -13,3 +13,8 @@ build:
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: docs/source/conf.py
+
+python:
+  install:
+    - method: pip
+      requirements: docs/requirements.txt

docs/requirements.txt

+sphinx-rtd-theme
\ No newline at end of file