docs: fix two inaccuracies found dogfooding #2338 (responsiveStyles keys, dataset include)

Browser/dogfood verification of the features documented in #2338 (booted app-showcase,
exercised at runtime) confirmed $search / searchableFields and multi-hop dataset joins
work as documented, but surfaced two doc inaccuracies in #2338 itself:

- page.mdx: responsiveStyles breakpoint keys are `large` / `medium` / `small` / `xsmall`
  (desktop-first — `large` is the unconditional base, the rest are max-width overrides)
  per ResponsiveStylesSchema, NOT the `xs/sm/md/lg/xl/2xl` axis (those are BreakpointName,
  used by layout columns/visibility/order). Verified against the served
  showcase_styling_gallery page metadata (key: `large`).
- analytics-datasets.mdx: multi-hop joins must be DECLARED in the dataset's `include`
  (joins are derived from include; only declared paths are joinable). Verified live: an
  inline dataset on showcase_invoice_line with include:['invoice.account'] + dotted-path
  dimension `invoice.account.industry` returns correctly-joined rows.

Docs-only.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: os-zhuang

content/docs/guides/analytics-datasets.mdx

@@ -135,10 +135,13 @@ capability the inline single-object query could never reach:
 // → revenue by account.region, joined + RLS-enforced per object.
 ```
 
-Joins follow **dotted relationship paths** and may traverse **multiple to-one hops**
-(ADR-0071) — e.g. `account.owner.region` walks the chain order → account → owner →
-region. The path is capped at **3 join hops (4 objects)** and is **to-one only**;
-to-many traversal is rejected (aggregate the many-side as its own measure instead).
+Joins are **derived from the dataset's `include` declarations**: list the relationship
+paths to join (`include: ['account', 'account.owner']`), and dimensions/measures then
+reference fields along them by **dotted path** (`account.owner.region`). Only declared
+paths are joinable. Paths may chain **multiple to-one hops** (ADR-0071 — declaring
+`account.owner` implicitly includes `account`), are capped at **3 join hops (4 objects)**,
+and are **to-one only**; to-many traversal is rejected (aggregate the many-side as its
+own measure instead).
 
 ## How it runs
 

content/docs/guides/metadata/page.mdx

@@ -136,7 +136,7 @@ Components are the building blocks placed inside regions.
 | `events` | `Record<string, string>` | optional | Event handlers (action expressions) |
 | `style` | `object` | optional | CSS styles |
 | `className` | `string` | optional | CSS class names |
-| `responsiveStyles` | `object` | optional | **Preferred** SDUI styling channel (ADR-0065): per-breakpoint scoped CSS-property maps keyed by `xs`/`sm`/`md`/`lg`/`xl`/`2xl`, compiled to id-scoped CSS at render. Prefer design tokens, e.g. `{ base: { padding: 'var(--space-6)' }, md: { padding: 'var(--space-8)' } }`. Use over ad-hoc `style`/`className` for metadata-authored pages. |
+| `responsiveStyles` | `object` | optional | **Preferred** SDUI styling channel (ADR-0065): **desktop-first** per-breakpoint scoped style maps, compiled to id-scoped CSS at render. Keys are `large` (the unconditional base), then `medium` / `small` / `xsmall` as `max-width` overrides. Prefer design tokens, e.g. `{ large: { padding: 'var(--space-8)' }, small: { padding: 'var(--space-4)' } }`. Use over ad-hoc `style` / `className` for metadata-authored pages. |
 | `visibility` | `string` | optional | Visibility predicate (CEL expression) |
 
 The `type` field is a union of the standard `PageComponentType` enum and any custom string. The standard (namespaced) component types include: