docs: datastore migration guide for mobile

[cadivus]

Addition to #8578

Description

Updates the DataStore-to-GraphQL migration guide to include backend changes required for a zero-downtime migration.

Problem

PR #8540 assumes clients can keep their Gen1 backend with DataStore/conflict resolution enabled. This is incorrect because:

• Soft-deleted items (_deleted: true) remain in DynamoDB
• Standard GraphQL clients (Apollo) receive ALL items including soft-deleted ones
• Without _deleted in the schema, clients cannot filter them out

Solution

This PR documents the correct migration approach:

1. Disable conflict resolution via amplify update api
2. Manually re-add _deleted: Boolean to the GraphQL schema (maps to existing DynamoDB data)
3. amplify push to apply changes
4. Update frontend clients to filter { _deleted: { ne: true } }
5. (Optional) Clean up soft-deleted items and remove _deleted field once all clients are migrated

Why this approach?

• Zero-downtime: backwards-compatible with both DataStore and Apollo clients
• No maintenance window required
• No race conditions with in-flight deletions
• Gradual migration path: clients can be migrated incrementally

Files changed

• src/directory/directory.mjs — Added navigation entries for the new migration guide pages
• src/pages/gen1/[platform]/.../migrate-from-datastore/index.mdx — Parent page explaining why backend changes are required
• src/pages/gen1/[platform]/.../migrate-from-datastore/disable-conflict-resolution/index.mdx — Step-by-step guide for disabling conflict resolution

Testing

• All 59 test suites pass (269 tests)
• yarn prebuild generates directory.json successfully with new entries

Related: #8540