Consolidate and update the Building a block editor guide

[ndiego]

Fixes: #52978

What?

This PR updates the "Building a custom block editor" How-to guide by consolidating Building a custom block editor and Tutorial: building a custom block editor into one doc. This streamlines the content and the sidebar.

The visuals in the tutorial were also updated to reflect 6.3, and the tutorial itself received a brief content edit. The following changes were made:

• The main intro to the guide was updated
• All instances of "we" and "our" were updated to "you"
• Add more links to relevant functions, packages, etc.
• Formatting was improved and callout sections were added where it made sense

Once this PR is merged, WordPress/wporg-developer#254 will also need to be merged to apply a redirect from the removed tutorial page to the main doc.

[github-actions]

Flaky tests detected in c90627e.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/5718306856
📝 Reported issues:

• [Flaky Test] Pick the additional variation in the inserted Columns block #47057 in specs/editor/plugins/block-variations.test.js

[mburridge]

LGTM

[ndiego]

The redirect PR has been merged, so now merging this.