Add RTD configuration files. (enthought#719)

* Add `.readthedocs.yaml`.

* Add `docs/requirements.txt`.

.readthedocs.yaml

@@ -0,0 +1,23 @@
+# Read the Docs configuration file
+# 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-24.04
+  tools:
+    python: "3.13"
+
+# Build documentation in the "docs/" directory with Sphinx
+sphinx:
+   configuration: docs/source/conf.py
+
+# TODO: Setup recommended configurations.
+# Optionally, 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/requirements.txt

@@ -0,0 +1,3 @@
+sphinx==5.3.0
+sphinx_rtd_theme==1.1.1
+sphinx-notfound-page==1.0.2