PR template: add before/after table

[ellatrix]

What?

Adds a Before/After table to the PR template in the screenshot section.

Why?

• I'm a bit tired of having to write out the table structure all the time.
• People may not be aware that it's possible to use tables to display screenshots side-by-side.

How?

Adds a Markdown table skeleton.

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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Co-authored-by: ellatrix <[email protected]>
Co-authored-by: t-hamano <[email protected]>

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[t-hamano]

If we go ahead with this PR, I feel a comment description may be necessary. One example is as follows. What do you think?

## Screenshots or screencast
<!-- If you would like to upload screenshots, please paste them in the table cells below, showing before and after this PR was applied. -->

|Before|After|
|-|-|
||| 

[ellatrix]

Or:

|Before|After|
|-|-|
|<!-- Before screenshot here -->|<!-- After screenshot here -->|

[t-hamano]

Personally, I think it would be easier to understand if there were both comments suggested by you and mine.

[t-hamano]

I've updated this PR according to this comment, and as I believe this PR still makes sense, I would like to approve it.

[github-actions]

Flaky tests detected in 9826fb8.
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/12095626471
📝 Reported issues:

• [Flaky Test] Redirection after template creation #60678 in /test/e2e/specs/site-editor/site-editor-url-navigation.spec.js

[ellatrix]

Thanks @t-hamano, I've added a comment, also indicating that it's merely a suggestion and not a requirement when adding screenshots.