Include docs around entity expiration time about docs-website

[danielgblanco]

Hi there! The problem came mainly from two areas:

1. Alerts firing on a Kubernetes pod getting OOM killed, for example. It's super useful when an entity is attached to an alert, as it takes service owners directly into the pod view and start debugging. If the pod was killed more than 4 hours ago, the entity will not be attached, making the owner have to go to logs or other metrics via the query explorer. Not necessarily an issue with the expiration time, but it'd be good for service owners to know when entities expire.
2. When using the Kubernetes cluster view, it'd be good for service owners to know what pods they can expect to find in those views (e.g. if they go back 6 hours their pod that was not there would not be found)

[github-actions]

Hi @danielgblanco 👋

Thank you for filing an issue! We'll triage your issue and let you know if we have questions, and then route it to the appropriate team so we can get it solved.

[zuluecho9]

@danielgblanco Thanks Daniel. Could you give us a little more detail about what the problem you were trying to solve was? Was it specifically for K8s data that you were having issues? Or was it more general, for other types of data outside K8s? Maybe you could explain the specific challenge that led you to reporting this (and we thank you for that).

[zuluecho9]

@danielgblanco Thank you, Daniel. I'm adding some info about this in this PR, if you'd like to check it out. #14101

[danielgblanco]

Thanks!