Create migration guide for PostgreSQL to Snowflake Postgres

[sfc-gh-dshavkani]

Summary

- Adds a new quickstart guide for migrating external PostgreSQL databases to Snowflake Postgres using the Cortex Code CLI
- Covers both **pg_dump/pg_restore** (< 150 GB, brief downtime) and **logical replication** (large DBs, minimal downtime) migration paths
- Demonstrates the open-source `postgres-migration-to-snowflake` Cortex Code skill that automates the full migration workflow

## Guide details

- **id:** `migrate-to-snowflake-postgres-using-cortex_code`
- **Authors:** Dharmendra Shavkani and Elizabeth Christensen
- **Categories:** `featured, getting-started, migration`
- **Tags:** `Getting Started, Migration, Postgres, Cortex Code, Snowflake Postgres, Logical Replication, pg_dump, pg_restore`
- **Source file:** `site/sfguides/src/migrate_to_snowflake_postgres_using_cortex_code/migrate_to_snowflake_postgres_using_cortex_code.md`

## Test plan

- [ ] Verify guide renders correctly at quickstarts.snowflake.com
- [ ] Confirm all code blocks have correct language specifiers
- [ ] Confirm all section durations sum correctly
- [ ] Verify links to Snowflake docs are valid

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🔗 Staging Preview Ready

Quickstart: migrate-to-snowflake-postgres-using-cortex-code
Language: en

👉 Preview URL: https://publish-p57963-e462098.adobeaemcloud.com/en/developers/guides/migrate-to-snowflake-postgres-using-cortex-code-a08773201729df60e3cbe9177480692eebf9786e

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[github-actions]

🔗 Staging Preview Ready

Quickstart: migrate-to-snowflake-postgres-using-cortex-code
Language: en

👉 Preview URL: https://publish-p57963-e462098.adobeaemcloud.com/en/developers/guides/migrate-to-snowflake-postgres-using-cortex-code-60f9389c67f4c9cf10bf96abc46503852cca7546

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