Improve pages about machine learning topics

## Problem

Information about ML topics is currently scattered.

- https://cratedb.com/docs/guide/topic/ml/
- https://cratedb.com/docs/guide/start/query/ai-integration.html

## Solution

Bundle/group/categorize relevant content appropriately, and provide better guidance than before.

## Ingredients

### Details
- GH-292
- GH-303
- GH-307
- GH-310
- GH-311
- GH-312
- GH-316

### Index
- GH-314

/cc @seut