docs is missing aggregation configs

**APM Server version** (`apm-server version`):

**Description of the problem including expected versus actual behavior**:

apm-server docs has no info on aggregation settings: https://www.elastic.co/guide/en/observability/current/apm-configuration-process.html

https://github.com/elastic/apm-server/blob/main/internal/beater/config/aggregation.go

**Steps to reproduce**:

look at the docs

**Provide logs (if relevant)**: