Vite: Move default cache dir to node_modules/.cache

[IanVS]

Issue: N/A

What I did

It's always kind of bugged me that the default storybook vite cache is not in node_modules/.cache. It is this way because the default vite cache is node_modules/.vite, but it means that those of us used to getting into a clean state by rm -rf node_modules/.cachce also need to know about this other special folder that needs to be cleaned out too.

This PR moves the default cache directory into .cache, following the law of least surprise. :)

I also added a note to the migration guide, though I don't expect this break anyone's projects.

Lastly, I cleaned up a section of the migration guide which previously had its table-of-contents entry removed, but the content itself was still there.

How to test

• Is this testable with Jest or Chromatic screenshots?
• Does this need a new example in the kitchen sink apps?
• Does this need an update to the documentation?

If your answer is yes to any of these, please make sure to include it in your PR.

[yannbf]

Thanks @IanVS !