[Docs RFC]Rewrite Migrate> Low-downtime: Dual-write and backfill as Migrate heavy insert workloads

[billy-the-fish]

Rewrite https://docs.timescale.com/migrate/latest/dual-write-and-backfill/ with 2 sections

• Download from the source database
• Upload to the target database

In each section use tabs to have imports from:

• TimescaleDB
• PostgreSQL
• Other databses

Use the template in https://github.com/timescale/docs/pull/3263/files as guidance.

Either move https://docs.timescale.com/migrate/latest/dual-write-and-backfill/timescaledb-backfill/ to the Migrate folder, or move this information to the repo where it comes from.