[DOCS] Rewrite aggs overview (#64318) (#64409)

- Replaces more abstract docs about object structure and values source with task-based examples.
- Relocates several sections from the current `misc.asciidoc` file.
- Alphabetically sorts agg categories in the nav.
- Removes the matrix agg family. Moves the stats matrix agg under the metric agg family

Co-authored-by: debadair <debadair@elastic.co>

Author: jrodewig