Docs: Argument Clinic: Restructure "Basic concepts and usage" (#106981)

Split "Basic concepts and usage" into:

- Reference
  - Terminology
  - CLI reference

- Background
  - Basic concepts

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>