fix: add link to migration guide

[317brian]

I'm not sure if I should be using the filepath or a URL. The same link to the migration guide on "Get started: Migration and backward compatibility" uses the URL

If you have an existing site that uses Docsy as a Git submodule, and you would like to update it to use Hugo Modules, follow our [migration guide](https://www.docsy.dev/docs/updating/convert-site-to-module/)

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[deining]

LGTM. Using the filepath is a good decision.

[deining]

Thanks for spotting and fixing this broken link!