Docs for SSH Keys: Refine pasting from clipboard wording

What does this MR do?

The docs for adding an SSH key prefer using the clipboard to avoid possible extra strings. This is a good thing, but the second step doesn't refer to this again. If you keep doing this step by step with context switches, you might not know what

pasting your public key in the Key field

means in terms of the clipboard mentioned before. Therefore I'd suggest to add a small reference to the clipboard which is the default suggested way. This keeps 1. and 2. aligned and users hopefully understand it better.

Rationale for proposing the change: Helping a user at https://forum.gitlab.com/t/remove-ssh-key-from-git-bash/32789 and overlooking myself what the clip command actually does, I'm usually using notepad for that.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

Refining the doc wording.

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team