Migrate RtD settings to config file

Read the Docs now requires all projects to use a settings file.
https://blog.readthedocs.com/migrate-configuration-v2/

Also add a requirements file for the docs packages.

Author: kadler

.readthedocs.yml

@@ -0,0 +1,20 @@
+# .readthedocs.yml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+build:
+  os: "ubuntu-22.04"
+  tools:
+    python: "3.9"
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+  configuration: docs/conf.py
+
+# Optionally set the version of Python and requirements required to build your docs
+python:
+  install:
+    - requirements: docs/requirements.txt

docs/requirements.txt

@@ -0,0 +1,23 @@
+alabaster==0.7.13
+Babel==2.13.0
+certifi>=2023.7.22
+charset-normalizer==3.3.0
+docutils==0.16
+idna==3.4
+imagesize==1.4.1
+Jinja2==3.0.3
+MarkupSafe==2.1.3
+packaging==23.2
+Pygments==2.16.1
+requests>=2.31.0
+snowballstemmer==2.2.0
+Sphinx==3.5.4
+sphinx-rtd-theme==1.3.0
+sphinxcontrib-applehelp==1.0.4
+sphinxcontrib-devhelp==1.0.2
+sphinxcontrib-htmlhelp==2.0.1
+sphinxcontrib-jquery==4.1
+sphinxcontrib-jsmath==1.0.1
+sphinxcontrib-qthelp==1.0.3
+sphinxcontrib-serializinghtml==1.1.5
+urllib3==2.0.6