Documentation: Add missing items to the Architecture page

[gziolo]

In the block editor handbook, some initial changes were applied in #49184 to the current structure of Architecture page after the feedback received from @Mamaduka.

What is missing on this page?

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It's also an excellent opportunity to discover missing key concepts related to the Gutenberg project. We have the following covered:

• Blocks
• Composability
• Data & Attributes
• Block Transforms
• Block Variations
• Reusable Blocks
• Patterns
• Templates
• Styles

[Mamaduka]

I think these two items would be a great addition to the page.

• RichText - I can't find any docs about architecture/design.
• History - how the editor records "edit" steps and handles undo/redo actions.

P.S. I'm planning to tackle the issues for the "History" feature. So I'll be able to write that page myself eventually, but having more context about previous decisions is always appreciated.

[juanmaguitar]

@ndiego

[talldan]

My thoughts, it could be an idea to structure this in terms of:

• Lead with a section for the pure block editor architecture, excluding WordPress-specific concepts.
• Top-level items about the WordPress implementation of the block editor (Data/Entities, our History implementation, Openverse?, general notes on extensibility)
• Project structure
• Some specific notes on each editor
  • Site editor (Templates, Template Parts, Styles, Layout, Query loop, Post Templates, Extensibility)
  • Post Editor (Post types, Extensibility)
  • Widget Editors (Sidebars, Block Widgets, Customizer integration)