docs: replace image to one which includes ca-certificates

[raweber42]

Checklist:

• [x] Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
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This PR improves the example given for installing CMP plugins in the documentation. The current image (busybox) does not include ca-certificates, which made the installation fail in my case. This will avoid users from having to debug on a very deep level as I had to. I am open for other image suggestions, of course! Closes [ISSUE #17885]

[raweber42]

It's my first PR of this kind, feel free to give me some tips on how to improve!