feat(v2): open external links in new tab by default

[jknoxville]

This changes the Link component to open external URLs in new tabs. This includes links in markdown in the form of [label](url), and also explicit use of the Link component.

You can override this behavior by passing target: '_self' as a prop when using Link.

Motivation

Docusaurus is already opinionated about doing this, for example the Navbar and
Footer do this already for external links. This is making it consistent across the entire site.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

cd website; yarn start;

• In browser navigate to http://localhost:3000/docs/design-principles and click on "Minimal Surface Area" - Verify it opens in new tab. Do the same on other pages.
• Verify anchor links continue to use the same tab.
• Verify internal links continue to use the same tab.
• Edit docs/design-principles.md to use Link instead of markdown syntax.
  • Verify it opens in a new tab.
• Edit docs/design-principles.md to add the target="_self" attribute.
  • Verify it opens in the same tab.

Related PRs

None

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[yangshun]

Thanks John! We have to add docs to specify this behavior but I'll do that for you.