[CI] Add a reminder to .gitmodules about pushing docsy tags (open-telemetry#4008)

Author: chalin

.gitmodules

@@ -2,6 +2,7 @@
 	path = themes/docsy
 	url = https://github.com/cncf/docsy.git
 	docsy-pin = v0.9.0
+	docsy-reminder = "Ensure that all tags from google/docsy are also present in cncf/docsy, otherwise add (push) them."
 [submodule "content-modules/opentelemetry-specification"]
 	path = content-modules/opentelemetry-specification
 	url = https://github.com/open-telemetry/opentelemetry-specification.git