Create a system-notes page stub

What does this MR do?

We reference system notes in various parts of our documentation, but we've never actually collated this information into one place. This merge request starts the process of documenting them.

• Placement of page? These notes appear on merge requests, issues, epics, issue boards, and incidents. I started out with doc/user/project/notes.md but I'm not strongly wedded to this location.
• Note creation? Can users CRUD system notes through the REST API or GraphQL?
• Review related code to determine what I can steal:
  • app/helpers/system_note_helper.rb
  • app/services/system_note_service.rb
  • https://docs.gitlab.com/ee/api/graphql/reference/index.html#systemnotemetadata
  • https://docs.gitlab.com/ee/api/graphql/reference/index.html#note
• Follow-up: add crosslinks back to this page, and add the page to the left nav.

Related to Document system notes used in merge requests (#372768 - closed) but does not close it

Pages that reference system notes

• doc/development/sidekiq/index.md
• doc/user/discussions/index.md
• doc/user/project/issue_board.md
• doc/user/project/quick_actions.md
• doc/user/project/merge_requests/versions.md
• doc/user/project/merge_requests/cherry_pick_changes.md
• doc/user/project/notes.md
• doc/user/project/issues/managing_issues.md
• doc/user/project/issues/confidential_issues.md
• doc/operations/incident_management/incidents.md
• doc/operations/incident_management/manage_incidents.md
• doc/operations/incident_management/escalation_policies.md
• doc/operations/incident_management/alerts.md
• doc/api/discussions.md
• doc/api/graphql/reference/index.md
• doc/api/notes.md
• doc/administration/instance_limits.md

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.