Update docs for confidential issues

What does this MR do?

• Update issue list screenshot for Indications of a confidential issue

• Update new confidential issue screenshot for Indications of a confidential issue

• Add documentation section about Confidentiality sidebar

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

Shouldn't be

Why was this MR needed?

Documentation

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
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/36352