docs: explain Sinon 20 fake server migration

[GreedyC]

Purpose (TL;DR) - mandatory

Fixes #2652 by adding the missing Sinon 20 migration section. Explain how existing XMLHttpRequest-based tests can use nise directly after the fake XHR and server APIs were unbundled.

The guide maps the removed APIs to their nise equivalents and explicitly calls out cleanup: fakes created directly through nise are not restored by a Sinon sandbox. It also distinguishes XMLHttpRequest from fetch.

How to verify - mandatory

1. npm ci --ignore-scripts and npm run build-artifacts in the repository root.
2. npm run lint and npx prettier --check docs/guides/migration.md.
3. In docs/, run npm ci --ignore-scripts, npm run docs:build and npm run test:docs.

These checks passed locally; the documentation tests reported 732 passing assertions across 251 suites. A separate nise installation was also used to verify a fake server response, the XHR factory/class mapping and explicit restoration. No library runtime files or dependencies are changed.

Checklist for author

• npm run lint passes
• References to standard library functions are cached. (Not applicable: documentation-only change.)

AI assistance: Codex helped prepare the guide and run validation.