Allow monitor script to be run without requiring snowplow-micro

What does this MR do and why?

Removes the need to run Snowplow Micro when using the Internal Events monitor, but warns the user that event level data is not available if Snowplow Micro is not available.

Also restructures our Docs to recommend using the Monitor clearly.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Make sure you have not set up Snowplow Micro. E.g. run gdk config set snowplow_micro.enabled false and gdk reconfigure before you restart GDK
2. Run rails runner scripts/internal_events/monitor.rb <your_event_name>. Internal events should still start and show a single table with the Metrics relevant to your event
3. Run gdk rails console and trigger your event with Gitlab::InternalEvents.track_event("<your_event_name>", user: User.first, project: Project.first). Metrics in monitor should be updated as expected.

Related to #461635 (closed)