Update systemd.mdx to remove tip to broken Katacoda link

[jasonrandrews]

The tip contains a link to Katacoda, which is now closed to the public.

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Name	Status	Preview	Comments	Updated (UTC)
parca-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Feb 5, 2024 5:39pm

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[kakkoyun]

@jasonrandrews Thanks for the fix. We wanted to migrate to https://killercoda.com/ but never got to it.

[jasonrandrews]

I realized this issue exists in many places in the documentation. If you have an alternative solution, I'm happy to close this PR which removed the broken link. I'm also happy to replace all instances with the new solution and test. Let me know. Thanks for the great software!

[kakkoyun]

@jasonrandrews We would be more than happy if we have tutorials working again with an alternative solution.

Feel free to contribute and let us know if you need help!