doc: Move settings to .readthedocs.yml

To follow the new best practice for ReadTheDocs, move docs build related
settings to the .readthedocs.yml config file.

.readthedocs.yml

@@ -0,0 +1,21 @@
+# .readthedocs.yaml
+# 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:
+   configuration: docs/conf.py
+
+# Optionally build your docs in additional formats such as PDF
+formats:
+   - pdf
+   - epub
+
+# Optionally set the version of Python and requirements required to build your docs
+python:
+   version: 3.8
+   install:
+   - requirements: requirements/doc.txt