[Monitoring Doc] Legacy and Metricbeat collection pages should point to agent-driven monitoring

Currently, when performing web searches for "Monitoring Logstash", the post popular landing pages are either "Monitoring Logstash with APIs", "Collect Logstash monitoring data with metricbeat", or "Monitoring Logstash (Legacy)".

This means that users may not be aware of agent-driven monitoring, the preferred method for Logstash monitoring. We should consider adding banners on these pages referring users to the agent-driven monitoring page.

Results from popular search engines:

Locate the “Monitoring Logstash with APIs,” “Collect Logstash monitoring data with metricbeat,” and “Monitoring Logstash (Legacy)” pages in the documentation, then identify the existing agent-driven monitoring page they should reference. Add banners to those pages and verify that each link clearly directs users to the preferred monitoring method.

Written by the indexing model from the issue text.