docs: configure RTD using config file

This gives us more visibility into the RTD settings, rather than the
settings being only visible to the account owning the docs (lukehsiao).

In addition, this treats warnings and errors so we can catch PR
documentation issues early. See #515.

.readthedocs.yml

@@ -1,5 +1,24 @@
+# .readthedocs.yml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+  builder: html
+  configuration: docs/conf.py
+  fail_on_warning: true
+
+formats:
+  - pdf
+  - epub
+
 build:
   image: latest
 
 python:
   version: 3.7
+  install:
+      - requirements: docs/requirements.txt