Document server-side rendering setup

[AyobamiH]

Summary

• add a canonical server-side rendering and React Server Components guide
• link the guide from the README and expose it under Storybook's Guides section
• document NHS.UK frontend styles, browser feature detection, hydration, RSC import patterns and progressive-enhancement checks

This is a documentation-only change: 4 files, 164 additions and no deletions.

Why

Version 6 added SSR support, but issue #269 was reopened because the setup and implementation guidance was still missing.

Validation

Validated the exact PR head, 66fc09d, in GitHub Actions run 33184406763:

• Node.js 24.19.0 and Yarn 4.14.1
• yarn install --immutable
• yarn lint
• yarn test --coverage --runInBand: 55 suites, 503 tests and 216 snapshots passed
• yarn build
• yarn build-storybook
• focused checks for the README link, Storybook guide integration, client boundaries and NHS.UK module loading
• git diff --check origin/main...HEAD

The upstream CI Build run is currently action_required and has not executed any jobs; it still requires maintainer approval.

Closes#269