ssh-add: added simplest command and split into platforms

pages/linux/ssh-add.md

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+# ssh-add
+
+> Manage loaded ssh keys in the ssh-agent.
+> Ensure that ssh-agent is up and running for the keys to be loaded in it.
+
+- Add the standard ssh key, e.g. {{id_rsa}}, to the ssh-agent:
+`ssh-add`
+
+- Add a specific new key to the ssh-agent:
+`ssh-add {{path/to/private_key}}`
+
+- List fingerprints of currently loaded keys:
+
+`ssh-add -l`
+
+- Delete a key from the ssh-agent:
+
+`ssh-add -d {{path/to/private_key}}`
+
+- Delete all currently loaded keys from the ssh-agent:
+
+`ssh-add -D`