feat(collections): replace intro section with collections block

[rbcorrales]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

This PR refactors the Collections intro template to use the Collections block for rendering instead of hardcoded HTML, to make the block and template output consistent.

There are a few improvements to the overall list layout of the Collections block to make it match the styling from the template part.

Closes NPPD-883.

How to test the changes in this Pull Request:

1. Verify intro section rendering

• Visit any single collection page
• Confirm intro section displays correctly (image, title, meta, excerpt, CTAs)

2. Perform the same tests from above on the archive page.
3. Test with numberOfCTAs= -1 and verify all CTAs display
4. Check styling consistency

• Compare the single collection page intro with the Collections block output
• Test responsive behavior on mobile/desktop

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[dkoo]

@rbcorrales: Approving based on the PR description and the changeset! Am I correct that the attributes for displaying the grid in the archives and the intro in the single collection template are hardcoded, but filterable through code? To test the archive with "numberOfCTAs" => -1 I filtered the attributes using newspack_collections_render_grid_attrs.

[rbcorrales]

@dkoo Correct, the block attributes are filterable. For the number of CTAs, we can later surface this option in the block editor, but for this PR, it was more to make the block serve as a drop-in replacement for the previous HTML template, which rendered all CTAs by default. I could have also hardcoded it to a large number so it will fit most use cases, but I think it's cleaner and more intentional this way 😄

Thanks!