MINOR: Improve docs by adding ToC links to Monitoring

My top 2 reasons for visiting the Kafka docs are to:
- View configurations
- View metrics

This PR aims to improve the user experience for viewing metrics:
- Add href links to the `Monitoring` section of the Table of Contents so users do not need to scroll or Ctrl/Cmd-F to find specific metric details (Monitoring section has grown large as more component & metrics are added)

Author: lu.kevin@berkeley.edu <kelu@paypal.com>

Reviewers: Viktor Somogyi <viktorsomogyi@gmail.com>, Manikumar Reddy <manikumar.reddy@gmail.com>

Closes #5511 from KevinLiLu/feature/minor-improve-docs