Make pane an optional region for PageLayout component

[vdepizzol]

What are you trying to accomplish?

This PR adds an expected functionality to PageLayout, making the pane region optional, and allowing single-column pages to also benefit from PageLayout's spacing structure.

I've also fixed two Storybook-related issues:

• Align PageLayout story spacing props with component
• Fix PageLayout storybook example imports

What approach did you choose and why?

PageLayout has two wrapping elements: PageLayout-wrapper and PageLayout-columns. wrapper takes care of vertical stack (header, content+pane, footer), while columns handles the horizontal the stack (content + pane).

For single column layouts, I've removed PageLayout-columns from the markup, leaving content as a direct sibling of header and footer.

Note that the hasPane prop from Storybook shouldn't translate 1:1 to ViewComponent or React APIs; the presence of compound components or slots should determine the region existence.

What should reviewers focus on?

Are additional changes needed?

• No, this PR should be ok to ship as is. 🚢

[changeset-bot]

🦋 Changeset detected

Latest commit: 9c896d4

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/css	Minor

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[jonrohan]

❤️ Love it, needing this for PageLayout propagation on dotcom

[langermank]

Love the helpful CSS comments!