[chore] add warning documentation around healthcheck extension (#29782)

This follows
open-telemetry/opentelemetry-collector#9065 (comment)

---------

Signed-off-by: Alex Boten <aboten@lightstep.com>
Co-authored-by: Pablo Baeyens <pbaeyens31+github@gmail.com>

extension/healthcheckextension/README.md

@@ -1,5 +1,14 @@
 # Health Check
 
+> ⚠️⚠️⚠️ **Warning** ⚠️⚠️⚠️
+>
+> The `check_collector_pipeline` feature of this extension is not working as expected. It
+> is recommended to not use the feature. The work to add a new version of the healthcheck extension
+> that relies on individual component status is in progress. To avoid breaking backwards compatibility,
+> the configuration will not be changed until the new extension is available as a replacement for this
+> one. See https://github.com/open-telemetry/opentelemetry-collector-contrib/issues/11780 for more
+> details.
+
 <!-- status autogenerated section -->
 | Status        |           |
 | ------------- |-----------|