Add docs on VTGate buffering for MoveTables and Reshard traffic switching

[mattlord]

Preview links:

• https://deploy-preview-1733--vitess.netlify.app/docs/20.0/reference/vreplication/movetables/
• https://deploy-preview-1733--vitess.netlify.app/docs/20.0/reference/vreplication/reshard/
• https://deploy-preview-1733--vitess.netlify.app/docs/20.0/reference/features/vtgate-buffering/

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[mattlord]

Ping on this @deepthi — obviously not urgent, but I know it's easy for website things to slip through the cracks. 🙂