Update and consolidate the "Building a custom block editor" documentation

[ndiego]

What problem does this address?

The Building a custom block editor doc and the accompanying tutorial, are the only information that currently exists in the Block Editor Handbook on how to use Gutenberg as a framework and development platform. These docs, while great, were written back in 2020, and a lot has changed since then. They need a thorough refresh.

What is your proposed solution?

The first step will be to consolidate Building a custom block editor and Tutorial: building a custom block editor into one doc. This will simplify content and sidebar:

Next, the visuals in the tutorial need to be updated, and the tutorial itself needs thorough testing to ensure it's still accurate and up-to-date.