Warn the user when visiting old documentation

This enhances our documentation to look for the file
/release_version.json in the root of our site. If found, and if it
contains a RELEASE_VERSION other than this version, then unhide a banner
warning about the stale documentation and linking to the current.

doc_src/conf.py

@@ -112,6 +112,9 @@ pygments_style = None
 html_theme_path = ["."]
 html_theme = "python_docs_theme"
 
+# Shared styles across all doc versions.
+html_css_files = ["/docs/shared/style.css"]
+
 # Don't add a weird "_sources" directory
 html_copy_source = False