Docs: Fix the Blueprint example of the Gutenberg PR preview

[ironnysh]

What is this PR doing?

Fixes the Playground example embedded in this documentation page, which didn't load.

How is the problem addressed?

1. Resolved the errors displayed in the browser console.
2. Updated the Blueprint based on the code of the official Previewer.
3. Replaced the PR loaded in the example.

Why step 3?

It seems that there's an unrelated problem with the specific PR used in the demo:

1. It doesn't work when loading it directly in the Gutenberg PR Previewer.
2. Other random PRs load correctly in the example.

Using 60819 can be a temporary workaround.

Testing Instructions

1. Run nx dev docs-site.
2. Go to this documentation page.
3. Scroll a bit, and click the Try it now button.

[adamziel]

Thank you @ironnysh! I merged and only noted this later – let's also update the captions to say PR 60819. I'm not sure what's wrong with 47739, but I hope it's not a problem with GitHub removing older artifacts as that would mean we'll have to update these docs periodically or find a different way of demonstrating the previewer. CC @bgrgicak – would you please check?

[ironnysh]

let's also update the captions to say PR 60819.

Sure. I'll open a new PR.

a problem with GitHub removing older artifacts

I did some tests with a bunch of random PRs (in different statuses) going back to 2017, and seems like that's indeed the problem.