HyperLogLog documentation

[anagrd-ai]

Add docs for the HLL functions, with callouts for support in the different dialects. Specifically, each DB supports a subset of HLL functions.

For all the details, see the design doc: https://docs.google.com/document/d/1acWaGvlPjFrdAh-gJ1bvulL8d2htaARqjX7a5tqWeL8/edit?tab=t.0

Dialect-specific HLL coding is tracked in issue:
malloydata/malloy#2054