Make deprecation notice of the spam checker doc more obvious (#10395)

changelog.d/10395.doc

+Make deprecation notice of the spam checker doc more obvious.

docs/spam_checker.md

-**Note: this page of the Synapse documentation is now deprecated. For up to date
+<h2 style="color:red">
+This page of the Synapse documentation is now deprecated. For up to date
 documentation on setting up or writing a spam checker module, please see
-[this page](https://matrix-org.github.io/synapse/develop/modules.html).**
+<a href="modules.md">this page</a>.
+</h2>
 
 # Handling spam in Synapse