docs(accordion): add nested accordion example

[stafferin]

Summary

Adds a "Nested" subsection to the Accordion docs page with a working example of an accordion nested inside another accordion's body. The example highlights the key gotcha: the nested .accordion-collapse elements must use data-bs-parent pointing at the nested container, not the outer one, so opening/closing nested items doesn't affect the parent accordion.

Closes #41895.

Test plan

• Build the docs site (npm run docs-serve) and visit the Accordion page
• Confirm the new "Nested" example renders between "Always open" and "Accessibility"
• Confirm parent items still toggle independently of nested items, and vice versa