Docs: explain tables in rounded containers

[nrps9909]

Description

• Document why opaque table cell backgrounds can cover a rounded parent container.
• Add a copy-ready .overflow-auto example that clips accented table backgrounds while preserving horizontal scrolling.
• Document --bs-table-bg: transparent as an alternative for plain tables, including when it is not sufficient.

Motivation & Context

Bootstrap 5.3's solid table cell backgrounds can obscure a wrapper's border radius. This documents both maintainer-endorsed workarounds from #41723 and clarifies the difference between plain and accented tables.

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 (not applicable: documentation-only change)
• All new and existing tests passed (809 JS assertions passed, but the local Chrome 151 Karma runner reported an unrelated full-page reload after completion; all documentation validations passed)

Live previews

• https://deploy-preview-42823--twbs-bootstrap.netlify.app/docs/5.3/content/tables/#tables-in-rounded-containers

Related issues

• Addresses v5.3 regression: table clips container's rounded border #41723.

Validation

• npm run lint
• npm run dist
• npm run docs-build
• npm run docs-vnu
• Prettier and CSpell checks for site/src/content/docs/content/tables.mdx
• Local rendered-page review in Chrome
• npm run js-test: 809/809 assertions passed; the local Chrome 151 Karma runner then reported a full-page reload environment error

Disclosure: Codex assisted with preparation; the source behavior, final diff, and rendered output were cross-checked locally.