Document converting integer columns to bigint

What does this MR do and why?

This MR adds a section to the development guides to describe the process of converting integer primary key to bigint without downtime.

http://main-ee-72475.178.62.207.141.nip.io/ee/development/avoiding_downtime_in_migrations.html#migrating-integer-primary-keys-to-bigint

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Related to #292988 (closed)