Expand documentation to include `user-data-defaults` and `user-data-overrides`

[stockholmux]

Path to page with content (do not include domain name):
/en/os/1.15.x/concepts/api-driven/

What content is incorrect?

Related to #125, the current documentation on user-data is very sparse and doesn't include additions from 1.15.0

What should the content say or communicate?

It should include include the new sources of user data baked into all variants for 1.15.0 (see bottlerocket-os/bottlerocket#3400)