Improve the Snowplow guide/docs introduction

The introduction to our Snowplow guide starts out by defining Snowplow and the Snowplow schema. We need to update this to better reflect our current state.

Current content

We have many definitions of Snowplow's schema. We have an active issue to standardize this schema including the following definitions:

• Frontend and backend taxonomy as listed below
• Structured event taxonomy
• Self describing events
• Iglu schema
• Snowplow authored events

Improvements

• Review and update the first list of "useful links"
• Update, reword or remove the "Snowplow schema" section

/cc @gitlab-org/growth/product-intelligence/engineers