Some links to documents don't target WebHome in nested migrations

Nested migrations don't produce documents that are not WebHome's. It doesn't make sense to produce links to non-WebHome pages.

This shouldn't impact too many people as this happens only in case of broken links, which means that either the target page indeed doesn't exist anyway, or link mapping was not properly used and links would likely be broken anyway.

This was spotted while working on CONFLUENCE-224.