Change the Listener documentation

[lfrancke]

Description

• Adds a more detailed explanation as to why we have the operator to: https://docs.stackable.tech/home/stable/listener-operator/
• Changes the usage to be way more focused on users of the SDP
• Change the ListenerClass documentation to be easier to read (subjective) and contain warnings about pinned pods and a few other things
• Contains an unrelated update to "slab" to avoid build failures due to https://rustsec.org/advisories/RUSTSEC-2025-0047

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

Author

• Links to generated (nightly) docs added
• Release note snippet added

Reviewer

• Documentation added or updated. Follows the style guide.
• Changelog updated

Acceptance

• Proper release label has been added
• Links to generated (nightly) docs added
• Release note snippet added

[maltesander]

LGTM overall, minor nitpicks.

[maltesander]

LGTM!

[sbernauer]

LGTM, only two small nitpicks