List supported aggregates

I think doing `sqrt((sum(power(value, 2)) - count(value)*power(avg(value),2)) / (count(value)-1))` instead of stddev(value) should work

## Why are we implementing it? (sales eng)
STDDEV doesn't work out of the box. The above example shows a workaround for the feature.

### What are the typical use cases?

## Communication goals (e.g. detailed howto vs orientation)

### Good locations for content in docs structure
SQL Workarounds

## How does this work? (devs)

### Example sql

### Corner cases, gotchas

### Are there relevant blog posts or outside documentation about the concept/feature?

### Link to relevant commits and regression tests if applicable