Add note on gitlab-managed clusters

What does this MR do?

As experienced with #31759 (closed) I thought I could just as well propose a little change in the documentation about the current behaviour.

The issue is that I apparently removed some resources on my gitlab managed kubernetes cluster which made it impossible (for gitlab) to re-deploy to the cluster. So a warning about this should be the minimal expected.

Curious if this will be merged, hope so!

Screenshots

Not relevant for this MR.

Does this MR meet the acceptance criteria?

Started to edit the page from the homepage itself, did not check any of the links below directly. Let me know if needed. Tried to be pragmatic here ;-).

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

Just a documentation update for the website.

Security

Not applicable.