Replace .bg-white with .bg-body in docs

Replace .bg-white with .bg-body in docs

Response to issue #32810

Use of the .bg-white class to define visual aspects of the examples
would be unintuitive for theme builders to test themes on.
This replaces instances of .bg-white with .bg-body in the examples.

- Edited examples to replace use .bg-white class with .bg-body class
  - product
  - offcanvas
  - pricing
  - cheatsheet
  - cheatsheet-rtl
- Edited utilities colors.md & shadows.md to include .bg-body class