Document how merchants can roll their own keys

[allendav]

Sometimes merchants inappropriately backup their WordPress database, including options, to public places like GitHub repositories. If done on a site running the Stripe plugin, this can expose Stripe keys. We should document how merchants can rectify this themselves and, if they can't, make it so they can.

Edit: We should document this for merchants using our Stripe Connect Standard platform as well as merchants who are not.

[reykjalin]

Marking this as High priority because having clear and easy to follow documentation on how to do this is important for merchants if they're in a situation where they need to roll their keys.

@allendav feel free to change the prioritization.