Add troubleshooting section to migration guide

Describe potential pitfalls and how to mitigate them.

A non-exhaustive list:

- [ ] Document pre-bundling for complex custom setups

Start by locating the migration guide in the repository and review its existing setup and troubleshooting content. Document the potential pitfalls of pre-bundling for complex custom setups and how to mitigate them; done means the guide contains a clear troubleshooting section covering this checklist item.

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