Add population sizing doc

[relud]

Description (optional)

• document how to estimate population size for a given targeting expression for Firefox on desktop or mobile

Issue that this pull request resolves

Fixes: mozilla/experimenter#14143

While I have run the project locally with my changes, it is my first time contributing to this repo and it would be be ideal for the reviewer to check into my branch and ensure the page is rendering as expected.

[jaredlockhart]

This is so exciting thank you @relud 🙏 Did we setup templated redash queries we should also link to?

[mikewilli]

Did we setup templated redash queries we should also link to?

This seems like a neat thing to link from the Experimenter audience page too, with the templates filled in based on the configuration.

[relud]

Did we setup templated redash queries we should also link to?

This seems like a neat thing to link from the Experimenter audience page too, with the templates filled in based on the configuration.

That will require automated translation of JEXL conditions to SQL, which I believe is planned as step towards automated pop sizing, and I think that's expected to be it's own thing.

[relud]

Did we setup templated redash queries we should also link to?

Added links