Fix docs build (rjsf-team#1729)

* Create requirements.docs.txt * Create .readthedocs.yml * Update .readthedocs.yml

Author: epicfaace

.readthedocs.yml

@@ -0,0 +1,11 @@
+version: 2
+
+mkdocs:
+ configuration: mkdocs.yml
+
+formats: all
+
+python:
+ version: 3.7
+ install:
+ - requirements: requirements.docs.txt

requirements.docs.txt

@@ -0,0 +1 @@
+mkdocs==1.0.4