Update Reified Relation Documentation

[mdroidian]

Motivation

• Introduce comprehensive documentation for the new stored (reified) relations feature and provide clear migration guidance from pattern-based relations.
• Surface the new docs in the Roam documentation navigation so users can discover stored relations and migration guidance easily.
• Inform users that pattern-based relations are being deprecated and point them to the new stored relations workflow and migration instructions.

Description

• Added apps/website/app/(docs)/docs/roam/pages/reified-relations.md which documents stored relations, multi-user behavior, implementation notes, and future direction.
• Added apps/website/app/(docs)/docs/roam/pages/migration-to-stored-relations.md which describes the migration flow, mixed-mode behavior, and edge cases.
• Updated apps/website/app/(docs)/docs/roam/navigation.ts to expose links for reified-relations and migration-to-stored-relations in the Roam docs navigation.
• Inserted deprecation notices into creating-discourse-relationships.md and relations-patterns.md and fixed internal cross-links to ./discourse-context-overlay and between the stored relations and migration pages.

Testing

• No automated tests were run because these are documentation-only changes.

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Codex Task

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[linear]

ENG-1405 Update Reified Relation Documentation

[supabase]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 4 additional findings.