Adding an Existing SSH Key to the SSH Agent in Documentation

[Marc-henryJ]

Problem:
Users setting up SSH access on a new machine may face issues connecting to Scalingo because their private key is not automatically loaded into the SSH agent.
This is a common issue, especially for those who have already generated an SSH key but forgot to add it to the agent. Currently, our documentation does not explicitly mention this step, which can lead to confusion.

Proposed Solution:
Add a section in the SSH setup documentation (in https://doc.scalingo.com/platform/getting-started/setup-ssh-linux) to guide users on adding an existing SSH key to their SSH agent.

Example :
Add an existing key

Ensure the SSH agent is running :
eval "$(ssh-agent -s)"

Command to add the key:
ssh-add ~/.ssh/id_rsa

Verification step:
ssh-add -l