Adds How to Extend the Editor guide to handbook

[dhruvikpatel18]

Closes: #64275

What?

This PR adds a new comprehensive guide "How to Extend the Editor" to the Block Editor Handbook's How-to Guides section. The guide provides developers with a clear overview of editor extensibility options and practical implementation examples.

Why?

While there are various articles and documentation pieces about extending the Editor, there wasn't a single, comprehensive guide that shows developers all the different ways they can extend the Editor. This guide fills that gap by:

• Providing a central resource for editor extensibility
• Offering practical, real-world examples
• Consolidating best practices
• Complementing the existing SlotFill documentation

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Co-authored-by: dhruvikpatel18 <[email protected]>
Co-authored-by: ndiego <[email protected]>

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