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

**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)