docs(collapse): add nested collapse example

[pavan-sh]

Description

Adds a Nested collapse example to the Collapse documentation (site/src/content/docs/components/collapse.mdx). The new section demonstrates a parent collapse containing a nested trigger and nested collapse content for progressive disclosure.

Motivation & Context

Bootstrap supports nested collapse triggers/content, but the docs didn’t include an example. This PR fills that documentation gap and makes the nested pattern easier to discover and copy. (Related: #41895)

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-42125--twbs-bootstrap.netlify.app/

Related issues

• Fixes Add an example of using the nested Collapse component #41895