Added readthedocs.yml

This makes (part of, see [*]) configuration versionable and also allows to
install dependencies from multiple requirements files.

[*]: https://docs.readthedocs.io/en/stable/config-file/v2.html#migrating-from-the-web-interface

PiperOrigin-RevId: 251243922
Change-Id: I6bc3fd89e9b72c4248f804f93a3b6f73d59f2fc8

readthedocs.yml

@@ -0,0 +1,16 @@
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+version: 2
+
+sphinx:
+  builder: html
+  configuration: docs/conf.py
+  fail_on_warning: false
+
+python:
+  version: 3.7
+  install:
+    - requirements: requirements.txt
+    - requirements: requirements-tf.txt
+    - requirements: docs/requirements.txt