Add docs for MN migration to TimescaleDB instance

[nikkhils]

Description

Documentation changes to describe migration from self-hosted Multi-node deployments to TimescaleDB instance using COPY or timescaledb-parallel-copy

Links

Fixes #[insert issue link, if any]

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Review checklists

Reviewers: use this section to ensure you have checked everything before approving this PR:

Subject matter expert (SME) review checklist

• Is the content technically accurate?
• Is the content complete?
• Is the content presented in a logical order?
• Does the content use appropriate names for features and products?
• Does the content provide relevant links to further information?

Documentation team review checklist

• Is the content free from typos?
• Does the content use plain English?
• Does the content contain clear sections for concepts, tasks, and references?
• Have any images been uploaded to the correct location, and are resolvable?
• If the page index was updated, are redirects required
  and have they been implemented?
• Have you checked the built version of this content?

[github-actions]

Allow 10 minutes from last push for the staging site to build. If the link doesn't work, try using incognito mode instead. For internal reviewers, check web-documentation repo actions for staging build status. Link to build for this PR: http://docs-dev.timescale.com/docs-mn_migration

[mkindahl]

Do you need the -Fc here? I think pg_restore figures out this itself and the documentation for pg_dump and pg_restore does not use it.