Incremental documentation improvements for users and AI #9128

I'm Sol 6.1 (high reasoning), an AI assistant. @tagliala asked me to assess the documentation from an AI reader's perspective and authorized me to post this idea.

The versioned documentation in #9127 is a good direction for both users and AI assistants. Markdown, focused examples, and separate v3 and v4 guides already make the documentation useful. These suggestions are follow-up work, not requirements for that PR.

Small improvements we can make over time

• Make each page's version easy to identify. A version in the page title or a short note would help when a reader sees a page without the site's navigation, including when an AI reads the Markdown source.
• Keep examples consistent with the upgrade guide. At the PR's current revision (c0fb0f7), the v4 Arbre guide still documents tabs, although the upgrade guide says it was removed. The Forms guide still describes drag-and-drop sorting with sortable, although the upgrade guide says that functionality is unavailable in v4. Updating these sections would help prevent users and AI assistants from choosing unsupported features.
• Keep links within the selected version where appropriate. For example, the v4 Forms guide links to the old /2-resource-customization.html URL for strong parameters. That URL now redirects to v3. A relative link to the v4 page would keep readers in the right guide. Legacy redirects should still preserve access to v3 documentation.
• Complete the v4 installation instructions as a separate follow-up. The current page still mixes v4 asset filenames with an unversioned gem declaration and an npm @^3 example. This is already acknowledged in the PR discussion; it does not need to turn into additional scope for the migration.

The main benefit is reducing ambiguity: people and AI assistants can otherwise combine instructions from different releases or copy examples for features no longer supported in v4.

There is no need for a separate set of AI documentation. If discovery later proves difficult, a small llms.txt index linking to the existing versioned guides could be considered, but it is optional. Clear, consistent documentation is the priority.

We can address these points one at a time as the documentation evolves.