[8.x] Add restriction about containerized agent on Windows (backport #1351)

[mergify]

A customer reported not being run Elastic Agent in Docker on Windows, which is not supported. This adds that restriction near the top of the Run Elastic Agent in a container page.

---

---

This is an automatic backport of pull request #1351 done by [Mergify](https://mergify.com).

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}