docs: add data migration scripts section

[SteelRazor47]

I finally had some free time to add this section on data migration scripts, closes #12738

I copied most of the description from the release notes from v2.3.0, as I found them to be well written, and I simply added a basic example to illustrate the function signature(taken from packages/medusa/src/migration-scripts/migrate-product-shipping-profile.ts).

Let me know if I should change anything. I made sure to run it locally to check that it looked ok(which took me a few minutes, you are supposed to have a .env in www/apps/book otherwise the build fails. Very simple and straightforward but it isn't mentioned in the docs contribution guide, should it be added?)

[changeset-bot]

⚠️ No Changeset found

Latest commit: b053d18

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[vercel]

Someone is attempting to deploy a commit to the medusajs Team on Vercel.

A member of the Team first needs to authorize it.

[shahednasser]

Thank you for your contribution! I've added comments for fixes and changes.

[SteelRazor47]

should be all done

[shahednasser]

LGTM thanks!

[SteelRazor47]

Thank you for the edits really, now it is much more understandable especially for new users. Always appreciate your work, the medusa docs are some of the best I've seen 👍