add link to useful convo (#10540)

i don't think the conversation at
https://github.com/certbot/certbot/pull/10495#discussion_r2699618989 is
urgent/important enough to make a github issue for it, but i also feel
like it's worth keeping a link around in case any devs have problems
with this code in the future. i think there was some good ideas in there

i don't think this PR requires two reviews

certbot-ci/src/certbot_integration_tests/utils/proxy.py

@@ -1,5 +1,10 @@
 #!/usr/bin/env python
 # pylint: disable=missing-module-docstring
+#
+# Note: If you're having trouble with this module, there was some discussion
+# about how it could be ripped out entirely at
+# https://github.com/certbot/certbot/pull/10495#discussion_r2699618989 that you
+# may want to read.
 
 import http.server as BaseHTTPServer
 import json