Add requirements.txt for docs

Signed-off-by: Dan Yeaw <dyeaw@ford.com>

.readthedocs.yml

@@ -4,7 +4,4 @@ sphinx:
   configuration: docs/conf.py
 python:
    install:
-      - method: pip
-        path: .
-        extra_requirements:
-          - docs
+     - requirements: docs/requirements.txt

docs/requirements.txt

@@ -0,0 +1,8 @@
+# Keep until RTD fully support pyproject.toml
+docutils==0.15.2
+Pygments==2.4.2
+recommonmark==0.6.0
+Sphinx==2.2.1
+sphinx-rtd-theme==0.4.3
+sphinxcontrib-websupport==1.1.2
+tomlkit==0.5.8