Docs: add blank target and noopener rel to footer external links

[julien-deramond]

Description

This PR adds target="_blank" and rel="noopener" to all external links in the documentation.

Note that subdomains of getbootstrap.com are considered internal links and are not affected by this change (like in the header).

Motivation & Context

External links are already using target="_blank" and rel="noopener" in the header for external links. This PR does the same thing for the footer for consistency and UX reasons.

Type of changes

• Enhancement (non-breaking change)

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
• (N/A) I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-38894--twbs-bootstrap.netlify.app/