MINOR: add missing section to TOC (#20305)

Add new group coordinator metrics section to TOC.

Reviewers: Chia-Ping Tsai <[email protected]>

Author: mjsax

docs/toc.html

@@ -153,6 +153,7 @@
                 <li><a href="#monitoring">6.7 Monitoring</a>
                     <ul>
                         <li><a href="#remote_jmx">Security Considerations for Remote Monitoring using JMX</a>
+                        <li><a href="#group_coordinator_monitoring">Group Coordinator Monitoring</a>
                         <li><a href="#tiered_storage_monitoring">Tiered Storage Monitoring</a>
                         <li><a href="#kraft_monitoring">KRaft Monitoring</a>
                         <li><a href="#selector_monitoring">Selector Monitoring</a>