Docs: Clarify that blockGap support depends on layout support

[andrewserong]

What?

A small update to developer docs to clarify that spacing.blockGap only works for blocks that use the layout block support.

Why?

As discussed in #53155 (comment) it isn't clear that spacing.blockGap depends on the layout block support. Now that the layout block support is stable and documented, let's connect the two a tiny bit more in the documentation.

How?

• Add a sentence to the What is blockGap docs to link to the layout block support.
• Update the comment for spacing.blockGap that it is for blocks that opt in to layout.

I'm sure we could also add more detailed info in the future, but hopefully for now this adds a little bit of clarity.

Testing Instructions

Proof read, and double check that the included link correctly links to the right section of the block supports documentation.

[github-actions]

Flaky tests detected in 078df84.
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/5734085674
📝 Reported issues:

• [Flaky Test] should load styles added through enqueue_block_assets #53088 in specs/editor/plugins/iframed-equeue-block-assets.test.js

[ramonjd]

Here, take a 🧇

Edit: read well to me. Thanks for the addition 😄

[tellthemachines]

LGTM, thanks!