docs(dataset-schema): distinguish top-level fields from views.*.transformation.fields

[DaveHanns]

Summary

The Dataset schema specification page defines two different keys named fields, but does not contrast them explicitly:

• Top-level fields — a JSON Schema object keyed by field name (under properties). Drives server-side validation, MCP schema discovery for AI agents, and the documented API contract.
• views.<viewId>.transformation.fields — a flat array of field name strings. Selects which columns appear in the Console Output tab and their order. Display-only.

Because the two keys share a name and the page introduces them in separate sections, it is easy to populate only the view-level array. Console then still renders the selected columns, so the mistake looks harmless — but validation, the MCP schema, and the API metadata have no schema to work from.

What this PR changes

Adds a short ### The two \fields` keyssubsection under## Schema components`. The subsection:

• Names both locations and tables their type and purpose side by side.
• Calls out the common mistake (populating only the view-level array).
• Links back to the top-of-page example (which already populates both) and forward to the fuller ## Fields example.

No content is removed; the existing structure is unchanged.

Reproducer for the confusion

Following the page in order, a reader hits the views.*.transformation.fields array before reaching the top-level fields section, and often writes only the view-level array. For example:

{
    "actorSpecification": 1,
    "views": {
        "overview": {
            "title": "Overview",
            "transformation": { "fields": ["title", "price"] },
            "display": { "component": "table" }
        }
    }
}

This validates and renders in Console, but the Actor exposes no field-level schema for validation, the MCP server, or the API contract that downstream agents rely on. The intended shape has both keys populated (as in the page's own top-of-file example), and the new subsection makes that requirement explicit up front.

Test plan

• Read the updated page top to bottom — the distinction between the two fields locations is clear after one pass.
• The top-of-page example under ## Schema components still shows both keys populated (unchanged), and the new subsection references it.
• pnpm start renders the new subsection with the table formatted correctly and the two intra-page links resolving.

Surfaced during an evaluation of Apify surfaces for agent-driven Actor development.

[apify-service-account]

✅ Preview for this PR (commit d1bebf2) is ready at https://pr-2733.preview.docs.apify.com (see action run).