feat(v2): open external links in new tab by default (#2780)

Docusaurus is already opinionated about doing this, for example the navbar and
footer do this already.

This changes the Link component to do the same. This includes links in markdown
in the form of `[label](url)`.

You can override this behaviour by passing the target: '_self' prop.

packages/docusaurus/src/client/exports/Link.tsx

@@ -84,7 +84,10 @@ function Link({isNavLink, ...props}: Props) {
 
   return !targetLink || !isInternal || targetLink.startsWith('#') ? (
     // eslint-disable-next-line jsx-a11y/anchor-has-content
-    <a {...props} href={targetLink} />
+    <a
+      {...(isInternal ? props : {target: '_blank', ...props})}
+      href={targetLink}
+    />
   ) : (
     <LinkComponent
       {...props}