Document the toast framework and mount the region in cmsui/publicui

[InteraktivPreuss]

• Add docs/how-to-guides/show-toasts.md and link it from the toctree
• Mount the global Toast region in the cmsui and publicui layouts so toasts triggered from editor and visitor-facing routes actually render
• Include Toast.css in both CSS bundles via the new layout/styles/cmsui.css and the updated layout/styles/publicui.css
• Add success/info/error variants in Toast.css using Quanta tokens
• Add an auto-dismiss countdown bar with prefers-reduced-motion support and region-level pause on hover or focus-within
• Expose showProgress on ToastItem and a dismiss utility; show() now returns the toast key and accepts per-call timeout/onClose
• Lower DEFAULT_TOAST_TIMEOUT_MS from 25 s to 6 s and normalize timeout<=0
• Add Storybook stories for the new variants and the showProgress flag
• Add per-package news entries

Refs #32.

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• I signed and returned the Plone Contributor Agreement, and received and accepted an invitation to join a team in the Plone GitHub organization.
• I verified there aren't other open pull requests for the same change.
• I followed the guidelines in Contributing to Volto.
• I succesfully ran code linting checks on my changes locally.
• I succesfully ran unit tests on my changes locally.
• I succesfully ran acceptance tests on my changes locally.
• If needed, I added new tests for my changes.
• If needed, I added documentation for my changes, either in the Storybook or narrative documentation.
• I included a change log entry in my commits.

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If your pull request closes an open issue, include the exact text below, immediately followed by the issue number. When your pull request gets merged, then that issue will close automatically.

Closes #32

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📚 Documentation preview 📚: https://plone-aurora--118.org.readthedocs.build/

[stevepiercy]

This is a great start. Thank you!

I'd like to move the explanation sections into a separate file under conceptual-guides/toasts.md and cross-reference as needed. What do you think?

[InteraktivPreuss]

@stevepiercy
Thanks for the review. I split the page in two as you suggested:

docs/conceptual-guides/toasts.md covers the architecture, the countdown bar, and the mount points. docs/how-to-guides/show-toasts.md is task-only now and links to the concept page at the top.

All your inline suggestions are applied wherever they landed. The Storybook link passes linkcheck and the docs build is clean.

Two places I deviated from your literal text:

1. Under "Render your own region" I kept "For a different layout" instead of "For different layout". Easy to drop the "a" if you want.
2. I extended "React Aria" to both intros, so they read "primitives from React Aria Components" instead of react-aria-components. Let me know if you'd rather keep the package name.

I also added an "Architecture" heading with two subsections in the concept file to break up the longer page. Let me know if that feels heavy.