Low-urgency need: Migration guide (e.g. in Readme.md) for upgrading from older (pre-vite) spec-up to current one

In TAAWG we have some docs/reports that all link to each other, but one is on older spec-up and the rest are on the new one. Trying to migrate piecemeal has been a little clunky, seemingly due to the way vite starts from and extends a pre-existing index.html file provided in the spec-up template/repo. I'm unclear on what exactly should be in `package.json` to make the spec render as intended in vite, because I keep getting "type must be module to bundle into index.html" despite setting the type to `module` in package.json (perhaps this is just my broken mental model of how npm works and unrelated to vite?).

In any case, once I figure it out (with or without help) it might help to add instructions for migrating old specs to vite-powered specup somewhere.