Update documentation with secure password encryption method

What does this MR do?

Updates documentation to use a secure form of password encryption for ssh keys.

Are there points in the code the reviewer needs to double check?

Nope.

Why was this MR needed?

This documentation comes up on a list of the top 10 Google search results for "how do I create an ssh key". As SSH's default password encryption method is insecure, and many users will be using your documentation to generate SSH keys, it is important we update public documentation to use the more secure methods.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?