Docs: Add migration guide for legacy Online Builder.

[filipsobol]

Suggested merge commit message (convention)

Docs: Add migration guide for legacy Online Builder.

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Additional information

For example – encountered issues, assumptions you had to make, other affected tickets, etc.

[Witoso]

I will add ZIP path to this branch, so please hold-off the merge.

[gorzelinski]

I'm guessing the suggestion above will resolve the problem with this fragment. The whole thing is rendered as a code snippet, but I'm guessing that wasn't the intention.

Unfortunately, it didn't solve the problem. But I changed the formatting, and snippets are rendered now as expected.