ci(sphinx): set READTHEDOCS variable in Jinja2 HTML context (#602)

docs/conf.py

@@ -33,6 +33,16 @@
 #
 # needs_sphinx = '1.0'
 
+# https://about.readthedocs.com/blog/2024/07/addons-by-default/
+# Define the canonical URL if you are using a custom domain on Read the Docs
+html_baseurl = os.environ.get("READTHEDOCS_CANONICAL_URL", "")
+
+# Tell Jinja2 templates the build is running on Read the Docs
+if os.environ.get("READTHEDOCS", "") == "True":
+    if "html_context" not in globals():
+        html_context = {}
+    html_context["READTHEDOCS"] = True
+
 # Do not warn on external images.
 suppress_warnings = ["image.nonlocal_uri"]