docs(journeys): document data injection block IDs as recommended (legacy forms deprecated)

[DeolaJ]

Summary

Updates the three public journey embedding docs so that data injection is documented around stable, journey-wide block IDs as the recommended way to prefill and target journey blocks, while keeping the legacy step-index + block-name forms documented as deprecated but still supported (backward-compatible). Block IDs are unique across the whole journey and unaffected by block renames or step reordering, so embeds keyed by block ID keep working when a journey is restructured.

Each Data Injection section now leads with a complete, recommended block-ID example (covering initialStepId, initialState keyed by block ID, and blocksDisplaySettings targeting by blockId), using consistent example values across all three transports so they line up:

• initialStepId: 'f0e1d2c3-b4a5-6789-0abc-def012345678'
• initialState: { 'b1f2c3d4-5e6f-7a8b-9c0d-1e2f3a4b5c6d': { city: 'Berlin' } }
• blocksDisplaySettings: [{ type: 'DISABLED', blockId: 'b1f2c3d4-5e6f-7a8b-9c0d-1e2f3a4b5c6d', blockFields: ['city'] }]

The deprecated step-index + block-name array form now appears only inside each file's existing deprecated :::note as a secondary example.

Each page also gains a short Data Injection (preview) builder subsection: the new Journey Builder tool lets authors visually pick blocks/fields, set pre-fill values and read-only blocks, choose the starting step, and copy the generated snippet — no hand-writing of block IDs. A GIF placeholder (../../static/img/journey-data-injection-builder.gif, following the repo's flat static/img convention) plus a <!-- TODO: replace with recording --> marker are included on all three pages for a recording to be dropped in later.

The option types remain backward-compatible unions from the previous commit — earlier forms were not removed:

• initialState?: Record<string, Record<string, unknown>> | Record<string, unknown>[] — recommended: an object keyed by block ID → block value; deprecated (legacy): an array indexed by step position, each entry keyed by block name.
• BlockDisplaySetting targets a block by blockId? (recommended) or the legacy blockName? + stepIndex? pair (deprecated).
• initialStepId? (recommended, stable) and initialStepIndex? (deprecated, 0-based) are both documented.

Files updated

• docs/journeys/embedding.md (legacy __epilot iframe embed) — data-injection section leads with a complete __epilot.init([{ ..., dataInjectionOptions: { ... } }]) block-ID example.
• docs/journeys/web-components.md (<epilot-journey> web component) — leads with a complete data-injection-options='{...}' JSON-attribute block-ID example (valid JSON).
• docs/journeys/sdk.md (@epilot/journey-embed-sdk) — leads with a complete $epilot.embed('123').asIframe().mode('full-screen').dataInjectionOptions({ ... }).append('#target') block-ID example.

Changelog

Each file keeps its ## Changelog entry dated 2026-06-11: data injection documents block IDs as the recommended form (initialState keyed by block ID, initialStepId, and blocksDisplaySettings targeting by blockId), resilient to block renames and step reordering; the legacy step-index + block-name forms remain supported but are deprecated.

Test plan

• Docs build / preview renders the three updated pages
• Code fences and the inline data-injection-options JSON in web-components.md are valid (verified to parse)
• Replace the journey-data-injection-builder.gif placeholder with the recorded GIF

Part of PROD-1469.