[lit-starter-js] [lit-starter-ts] Update starter template READMEs

[pixelbandito]

• Change github pages branch name from "master" to "main"
• Change npm run docs:watch to npm run docs:gen:watch

[changeset-bot]

🦋 Changeset detected

Latest commit: 0c3c7e2

The changes in this PR will be included in the next version bump.

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[augustjk]

Thanks!
Could you run npx changeset --empty from project root and commit the empty file for CI? I would do it myself but you don't have the maintainers are allowed to make edits to PR enabled.

[pixelbandito]

@augustjk Done - that look ok?

[augustjk]

Awesome, thanks!