Set html_baseurl and html_context from environment variables

conf.py

@@ -6,6 +6,7 @@
 # full list see the documentation:
 # http://www.sphinx-doc.org/en/master/config
 
+import os
 from pathlib import Path
 import sys
 
@@ -90,6 +91,15 @@
 #
 # html_theme_options = {}
 
+# 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
+
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".