Add an Extend page with a community plugin registry - #42860

Changed pages include:

• index.html - view
• 404.html - view
• components/index.html - view
• content/index.html - view
• customize/index.html - view
• docs/index.html - view
• docsref/index.html - view
• examples/index.html - view
• forms/index.html - view
• getting-started/index.html - view

There are more changes not included in the list