Add docs for 'test-features'

[autoantwort]

@ras0219-msft edit:
Feature: microsoft/vcpkg-tool#802

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[AugP]

Converted this to draft for now until tool PR is merged.

[dg0yt]

It could make sense to briefly recommend a configuration with a compiler cache. There is a lot repetition in compilation commands over different feature combinations.

[autoantwort]

I have never used one. But maybe this should be an own article that then gets linked from here.