docs: mention Valibot as validation library option in forms guides (#97468)
The forms guides recommend validating with a library like Zod. This
broadens the wording to also mention Valibot, which covers the same use
case with a smaller bundle footprint. The authentication guide already
lists Zod or Yup, so Valibot is added to that list as well. The existing
Zod examples are unchanged. I'm the author of Valibot.
- Ran `prettier --check` on the touched files with the repo's pinned
version.