Docs: update react / reactive store guides to use twoslash examples

The docs PRs for the react stack (https://github.com/anza-xyz/kit/pull/1779, https://github.com/anza-xyz/kit/pull/1782) currently use ` ```ts` for their examples. This is not typechecked in the same way as `ts twoslash` and does not support the nice cut before syntax.

This is because docs uses a published Kit version, and we haven't yet published a version with the new code/changes.

When we do, and we update docs to 7.x, we should do a quick sweep and move most of these to `ts twoslash`.