docs: clarify Windows build status

[zuercher]

Per #28588, Windows builds are unsupported. Make that clear in the documentation. Additionally, remove references to the Windows docker image, which has not been updated since 1.28.0 with the intention of not directing Windows operators towards a version missing important security updates.

Risk Level: low, docs only Testing: n/a Docs Changes: yes Release Notes: n/a Platform Specific Features: n/a

[repokitteh-read-only[bot]]

Docs for this Pull Request will be rendered here:

https://storage.googleapis.com/envoy-pr/34328/docs/index.html

The docs are (re-)rendered each time the CI envoy-presubmit (precheck docs) job completes.

:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/34328 was opened by zuercher.

see: more, trace.

[phlax]

/backport