Revamp the `Layouts` page

[vy]

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

• Outline:
  • What is a layout?
    • List layouts and link to the associated plugin reference page
  • Layouts with dedicated pages
    • Pattern Layout
      • Fix PatternLayout unsuitable for production purposes #2518
    • JSON Template Layout
  • Extending layouts
    • Difference between Encoder and Layout
    • Note about garbage-free methods
• Address Document 2.x module/feature deprecation #1950
• Target 2.x, port to main