[REQUEST]: Update documentation about updating Elastic Agents in air-gapped environments

### Description

It would be interesting to update the documentation to make it clear which Elastic Agent versions are available in air-gapped environments.

These air-gapped environments cannot check which agent versions are actually available via the Elastic site, so Kibana/Fleet only could show the versions that were available at the time Kibana was built for that specific release.

For instance, in the following scenario where the Elastic stack is running, if an Elastic Agent is tried to be updated to 8.17.9 version is shown as not recommended :

Could it be for instance dismiss that warning (not recommended) if the environment is air-gapped?

### Resources

Related resources about Fleet used in air-gapped environments:
- https://www.elastic.co/docs/reference/fleet/air-gapped

### Collaboration

The documentation team will investigate the issue and create the initial content.

### Point of contact.

**Main contact:** @elastic/fleet @criamico