Document Profile Field customization

[djuber]

Explain the changes

Profile Fields have been added and enabled to the admin area, but there's no documentation for this page yet.

Suggested changes

Describe the relationship between field groups and fields, the types of fields available, and the different "target areas" (sidebar, main profile header, etc), so creators can confidently make decisions about these fields.

Additional context

Finalizes work in https://github.com/forem/forem-internal-eng/issues/352