docs: Add .readthedocs.yaml

Use Python 3.8 and pin the version of mkdocs.

Signed-off-by:  Eric Callahan <arksine.code@gmail.com>

.readthedocs.yaml

@@ -0,0 +1,10 @@
+version: 2
+
+mkdocs:
+  configuration: mkdocs.yml
+  fail_on_warning: false
+
+python:
+  version: 3.8
+  install:
+    - requirements: docs/doc-requirements.txt

docs/doc-requirements.txt

@@ -0,0 +1 @@
+mkdocs==1.1.2