docs(ssh-key): add synopsis and examples to ssh-key commands

Description

Adds Long (Synopsis) fields and improved Examples to the ssh-key command and its subcommands, as part of the style guide audit identified in #8263 (closed).

Each Synopsis explains the command's default behavior and key flags, without repeating the Short description. The list Examples section is annotated with # comment lines describing each invocation.

Commands updated:

  • ssh-key
  • ssh-key delete
  • ssh-key get
  • ssh-key list

Content follows the documentation style guide and the GitLab CLI (glab) documentation style guide.

Documentation

If this MR adds or updates CLI command documentation, follow the conventions in the GitLab CLI (glab) documentation style guide, run make gen-docs, and commit the updated files in docs/source/.

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