Doc: Section for "this application has no SSH instance you can connect to"

[PLhuillery]

Checking for existing issue

• I have checked for existing issues

Describe the content you'd like to see

What

When connecting with SSH to a running instance using Clever CLI, users can often encounter the following:

Error: Failed to choose instance: 'Error while running choice script: Error: This application has no instances you can ssh to'
Connection to sshgateway-clevercloud-customers.services.clever-cloud.com closed.

Why

The issue is actually an access right issues, and is related to having the wrong SSH key most of the time.
There is no info in FAQ or the page about connecting to running instances using SSH

What I'd like to see:

A mention to check that the SSH keys from the logging machine should match the one in the Clever Cloud org profile.