build: Manually pull some RTD Context.

See https://about.readthedocs.com/blog/2024/07/addons-by-default/ for details but
essentially RTD is changing how it's building docs and this will let us handle the
change gracefully.

docs/conf.py

@@ -258,6 +258,16 @@ epub_title = project
 epub_exclude_files = ['search.html']
 
 
+# -- Read the Docs Specific Configuration
+# 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
+
 # -- Extension configuration -------------------------------------------------
 
 # -- Options for intersphinx extension ---------------------------------------