Add A/B test significance calculator

[ivanagas]

Strapline

Explain the idea in a sentence or two

Add an A/B test significance calculator to posthog.com. Similar to: https://abtestguide.com/calc/

Why should we do it?

how it will it be useful or interesting for readers/viewers

A/B test significance calculators get a lot of search traffic. People ask about A/B test significance a lot. Would be a cool add.