Adds manual migration info to the Migration page

[momo-the-monster]

We'll need to create, deploy and link the World and UdonSharp Unity Package files described in this new section before publishing.

[github-actions[bot]]

🚀 Deployed on https://63f00af0eeaea514228d2335--vrc-udonsharp.netlify.app

[momo-the-monster]

@MerlinVR - thanks for the heads-up on that issue with #101, I hadn't seen your comments there yet. Yes, the Resolver package adds the ability to right-click a package directory in your Project view and select "Export VPM Package", which will make a package named for the id and version of the target package and place it in a new directory called "Exports" at the root of the project.