Query Loop block: Clarify explanation around query loop variation example

[artemiomorales]

What?

This PR clarifies that specifying the layout of a query loop block variation is needed in order for the variation to function properly.

Why?

Currently, the documentation is not clear that the Customize your variation layout step is needed — and if this is not done, the variation defaults to allowing the user to pick a layout in the UI, which overwrites the variation's attributes.

How?

It adds numbers to the headings under Extending the block with variations and also adds a clarifying sentence to ensure users follow the next step.

Testing Instructions

N/A

Testing Instructions for Keyboard

N/A

[github-actions]

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[artemiomorales]

I'm not sure that this PR will completely clarify the explanation, though I believe it will help. Any thoughts appreciated! 🙏

[github-actions]

Flaky tests detected in 9933b92.
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/9537974145
📝 Reported issues:

• [Flaky Test] Navigates from items list to preview via TAB, and vice versa #62434 in /test/e2e/specs/site-editor/dataviews-list-layout-keyboard.spec.js

[juanmaguitar]

Thanks for the PR @artemiomorales!
The changes look good and clarify the prerequisites for a customized query loop to work properly a bit more.

To provide even more clarity over these "requisites" I would also add just before...

Let's go on a journey, for example, of setting up a variation for a plugin ...

Something along the following lines:

In order to have a Query Loop variation properly working we'll need to 
- register the block variation for the `core/query` block with some default values
- define a layout for the block variation
- use the `namespace` attribute in the `isActive` block variation property

To provide an overview of what's explained next in more detail to ensure the block variation works as expected.