Corrected links/usernames in performance guide

What does this MR do?

This fixes two broken links in the performance guide and removes the mention of Josh as he no longer works for GitLab.

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

No.

Why was this MR needed?

The guide contained two broken links, and Josh no longer works for GitLab.

What are the relevant issue numbers?

None.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together