docs(visual-element): strip Unity API teaching, keep before/after tables

- drop scenario intros, code comments and paragraphs that explain Unity behaviour
- remove composed examples and per-element demo chains; unfold API spoilers
- rewrite custom USS section as a TryGetByEnum before/after table
- fill empty BaseField / IMixedValueSupport spoilers, drop the Next steps block

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: VPDPersonal