Add required Read the Docs v2 config file, switch to RTD theme

All Read the Docs (RTD) projects must have a configuration file v2 beginning September 25, 2023, or their documentation will no longer build on RTD servers. https://blog.readthedocs.com/migrate-configuration-v2/

RTD changed the default theme to Alabaster. Change back to the RTD theme. readthedocs/readthedocs.org#10692

.readthedocs.yaml

@@ -0,0 +1,37 @@
+# .readthedocs.yaml
+# Read the Docs configuration file for Sphinx projects
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+# Set the OS, Python version, and other tools you might need
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.11"
+    # You can also specify other tool versions:
+    # nodejs: "20"
+    # rust: "1.70"
+    # golang: "1.20"
+
+# Build documentation in the "docs/" directory with Sphinx
+sphinx:
+  configuration: docs/conf.py
+  # You can configure Sphinx to use a different builder, for instance use the dirhtml builder for simpler URLs
+  # builder: "dirhtml"
+  # Fail on all warnings to avoid broken references
+  # fail_on_warning: true
+
+# (Optional) Build docs in additional formats; Scout has historically offered PDF, ePub, and HTML
+formats:
+   - pdf
+   - epub
+   - htmlzip
+
+# Optional but recommended, declare the Python requirements required
+# to build your documentation
+# See https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
+# python:
+#    install:
+#    - requirements: docs/requirements.txt

docs/conf.py

@@ -136,6 +136,9 @@
         html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
     except ImportError:
         html_theme = 'alabaster'
+else:
+    extensions = ["sphinx_rtd_theme", ]
+    html_theme = "sphinx_rtd_theme"
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the