az sshkey: add page

[sdahlbac]

• The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• The page(s) have at most 8 examples.
• The page description(s) have links to documentation or a homepage.
• The page(s) follow the content guidelines.
• The PR title conforms to the recommended templates.
• Version of the command being documented (if known):

az version

{
  "azure-cli": "2.40.0",
  "azure-cli-core": "2.40.0",
  "azure-cli-telemetry": "1.0.8",
  "extensions": {
    "rdbms-connect": "1.0.2",
    "ssh": "1.1.2"
  }
}

[sdahlbac]

For #6141