docs: replace image to one which includes ca-certificates

[raweber42]

Checklist:

• 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.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Toolchain Guide
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates? Yes
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged. NOT NEEDED
• My build is green (troubleshooting builds). NOT NEEDED
• My new feature complies with the feature status guidelines. NOT NEEDED
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

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!