docs(nuxt-better-auth): standardize secondaryStorage guidance

[onmax]

Summary

• add an opinionated baseline for NuxtHub session storage strategy
• document clear decision checklist for when to enable secondaryStorage
• clarify SSR/session invalidation behavior and consistency caveats
• add rollout and rollback playbooks
• include explicit DB-default and KV-secondary config examples

Closes #137

[pkg-pr-new]

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npm i https://pkg.pr.new/nuxt-modules/better-auth/@onmax/nuxt-better-auth@142

commit: fc44e4c