Add User Account dropin docs

[keharper]

This PR adds user account dropin docs.

[jeff-matthews]

Looks good @keharper.

I only made superficial editorial suggestions. Specifically where the approved naming convention applies to "dropins".

Approved convention from internal wiki:

• "Drop-in components" is preferred over "DropIns."
  the name structure [Functional Name] Drop-in is correct for individual drop-ins. Acronyms should not be used as part of the name. (from Branding)

• "Cart drop-in component", once established, can be written as, "the cart component" or "the cart").

[KostiantynFandeliuk]

Looks good to me. Thank you.