fix(website): give Homebrew users an uninstall command, share the link markup

The macOS install route recommends Homebrew, but the uninstall section only
ever offered `cargo uninstall`, so anyone who followed the recommended route
had no documented way to remove the binary. Adds `brew uninstall`.

install.tsx also had ReleasesLink defined to wrap an external anchor, then
inlined that same anchor four more times anyway - including one that rebuilt
the releases link by hand, byte for byte, 290 lines below the component that
already rendered it. ExtLink takes the href and covers all six sites here and
in community.tsx. Rendered markup is unchanged.

website/src/components/ui/ext-link.tsx

@@ -0,0 +1,15 @@
+interface ExtLinkProps {
+  href: string
+  children: React.ReactNode
+}
+
+export const ExtLink = ({ href, children }: ExtLinkProps) => (
+  <a
+    href={href}
+    target="_blank"
+    rel="noopener noreferrer"
+    className="text-link hover:text-link-hover"
+  >
+    {children}
+  </a>
+)