Added Benchmark Section to sched-ext.md

[GameBoyNoob]

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[ptr1337]

As discussed in the chat, lets enhance this to an own page, which also includes information's related to vapormark.

[1Naim]

Hello, are you still working on this? Since your initial PR, there has been some additions to the CachyOS benchmarking scene that it needs to be in its own page to not convolute the sched-ext page. CachyOS now comes with its own benchmark tool which was forked from mini-benchmarker from your initial PR. Besides that, there is also vapormark by Igalia and currently the most used way of benchmarking, logging FPS with MangoHUD and parsing the logged content to https://flightlesssomething.ambrosia.one/benchmarks