[4.0.x] Fixed #33247 -- Added configuration for Read The Docs.

Co-authored-by: Andrew Neitsch <andrew@neitsch.ca>

Backport of 0da7a2e9dab81b622a2000536c6a96de7f46e237 from main

.readthedocs.yml

@@ -0,0 +1,18 @@
+# Configuration for the Read The Docs (RTD) builds of the documentation.
+# Ref: https://docs.readthedocs.io/en/stable/config-file/v2.html
+# Note python.install.requirements is not currently required, as Sphinx is
+# preinstalled and spelling checks not performed by RTD.
+version: 2
+
+build:
+  os: ubuntu-20.04
+  tools:
+    python: "3.10"
+
+sphinx:
+  configuration: docs/conf.py
+
+formats:
+  - epub
+  - pdf
+  - htmlzip