feat(docs): adds operator documentation for adding a data migration

[aristidesstaffieri]

What

Adds documentation for data migrations targeted at wallet backend operators.
Contains overview of the steps, examples of changes needed at each step, explanations of how/when to run each process, and a completion checklist.

Why

Operators need detailed instructions to setup and perform a data migration.

Known limitations

N/A

Issue that this PR addresses

N/A

Checklist

PR Structure

• It is not possible to break this PR down into smaller PRs.
• This PR does not mix refactoring changes with feature changes.
• This PR's title starts with name of package that is most changed in the PR, or all if the changes are broad or impact many packages.

Thoroughness

• This PR adds tests for the new functionality or fixes.
• All updated queries have been tested (refer to this check if the data set returned by the updated query is expected to be same as the original one).

Release

• This is not a breaking change.
• This is ready to be tested in development.
• The new functionality is gated with a feature flag if this is not ready for production.