docs: create client-side fetching guide (#96341)
## What changed
- Expands the SPA data-fetching guidance with focused SWR and TanStack
Query guides.
- Covers client-only fetching, optional Server Component seeding, shared
cache identities, and optimistic mutations.
- Explains how the Next.js server and client caches relate to
library-owned browser caches, including SWR `preload` and TanStack Query
hydration with Cache Components.
- Aligns the examples with `next-spa-patterns` and the team chat demo.
## Validation
- Prettier and diff checks pass for the updated guides.
- `next-spa-patterns` passes TypeScript checks.
(cherry picked from commit 7cb68c12828a758492ea54251393b4f988aecd6e)
