Add a guide for root types

[gorzelinski]

🚀 Summary

Adds a new "Root types" guide to the Getting started / Setup section covering the two built-in root model element types ($root and $inlineRoot).

---

📌 Related issues

• Closes https://github.com/ckeditor/ckeditor5-commercial/issues/9930

---

💡 Additional information

Optional: Notes on decisions, edge cases, or anything helpful for reviewers.

---

🧾 Checklists

Use the following checklists to ensure important areas were not overlooked.
This does not apply to feature-branch merges.
If an item is not relevant to this type of change, simply leave it unchecked.

Author checklist

• Is the changelog entry intentionally omitted?
• Is the change backward-compatible?
• Have you considered the impact on different editor setups and core interactions? (e.g., classic/inline/multi-root/many editors, typing, selection, paste, tables, lists, images, collaboration, pagination)
• Has the change been manually verified in the relevant setups?
• Does this change affect any of the above?
• Is performance impacted?
• Is accessibility affected?
• Have tests been added that fail without this change (against regression)?
• Have the API documentation, guides, feature digest, and related feature sections been updated where needed?
• Have metadata files (ckeditor5-metadata.json) been updated if needed?
• Are there any changes the team should be informed about (e.g. architectural, difficult to revert in future versions or having impact on other features)?
• Were these changes documented (in Logbook)?

Reviewer checklist

• PR description explains the changes and the chosen approach (especially when performance, API, or UX is affected).
• The changelog entry is clear, user‑ or integrator-facing, and it describes any breaking changes.
• All new external dependencies have been approved and mentioned in LICENSE.md (if any).
• All human-readable, translateable strings in this PR been introduced using t() (if any).
• I manually verified the change (e.g., in manual tests or documentation).
• The target branch is correct.

[niegowski]

I'd also mention and show examples for defining the DOM element for root as default div is not the best choice for inline roots as most probably it could be a h1. This is accepted as a different form of RootConfig#element where integrator passes a name or definition of the view/dom element instead of an actual DOM element. More details in RFC and #20146

[niegowski]

I'm not sure of the naming. Maybe it should be sth like a container? I'm not sure.

[gorzelinski]

Container may work. I also thought about:

• Standard
• Default
• Rich

WDYT?

[gorzelinski]

I've gone with the "standard" option. "Default" or "rich" sounds a bit generic. "Container" sounds better but may be confused with the $container (schema concept).