Revamp the `Plugins` page

[vy]

As agreed in the site structure document, revamp the Plugins page such that

• Outline:
  • Show examples (@PluginElement, @PluginAttribute, @PluginFactory, builder, etc.)
  • Link to Layouts > JSON Template Layout > Extending
  • Link to Layouts > Pattern Layout > Extending
• Target 2.x, port to main

[vy]

Fixed by #2696.