docs(rsc): document stable client chunks

[hugosmoreira]

Description

Documents the Vite 8/Rolldown configuration for keeping client chunks stable and applies it to the plugin-rsc basic example while preserving the example's Vite 7 CI coverage.

The updated documentation and example:

• use build.rolldownOptions.output.codeSplitting.groups
• group React, React DOM, and the RSC browser runtime into lib-react
• separate Vite's preload helper into lib-vite
• use a functional group test so CommonJS proxy module IDs are still matched
• detect Rolldown in the basic example and fall back to rollupOptions.output.manualChunks under Vite 7

Closes #865

Validation

• pnpm -C packages/plugin-rsc build
• pnpm -C packages/plugin-react build
• pnpm -C packages/plugin-rsc tsc
• pnpm -C packages/plugin-rsc/examples/basic build
• oxfmt --check packages/plugin-rsc/README.md packages/plugin-rsc/examples/basic/vite.config.ts

[hi-ogawa]

The document should be fore Vite 8 rolldown.

• https://vite.dev/guide/migration#removed-object-form-build-rollupoptions-output-manualchunks-and-deprecate-function-form-one
• https://rolldown.rs/in-depth/manual-code-splitting

Our examples/basic/vite.config.ts should also use that when running Vite 8.