Docs: State the incoming email prerequisite for convert_to_ticket

What does this MR do?

States the incoming email prerequisite for the /convert_to_ticket quick action and the Service Desk settings section, on the three pages where a reader meets them.

On an instance where incoming email is not set up with wildcard support (email sub-addressing or a catch-all mailbox), ServiceDesk.supported? is false:

• The Service Desk section in Settings > General never renders: app/views/projects/_service_desk_settings.html.haml#L1 is return unless ::ServiceDesk.supported?.
• The /convert_to_ticket quick action is unavailable, because its condition checks ServiceDesk.enabled?: lib/gitlab/quick_actions/issue_actions.rb#L271 (gating introduced in !187684 (merged)).

None of the three pages that document the command or the settings section state this. A reader on a self-managed instance without incoming email can reasonably conclude from the current docs that /convert_to_ticket creates tickets without any email configuration, then finds the command and the settings section absent, with no explanation on any of the pages. A customer administrator reached that conclusion, which is how the omission surfaced.

Changes:

• doc/user/project/quick_actions.md: add the availability condition to the convert_to_ticket reference entry, which is where users first meet the command.
• doc/user/project/service_desk/using_service_desk.md: add prerequisites to the "Create a Service Desk ticket in GitLab UI" and "Convert a regular issue to a Service Desk ticket" sections.
• doc/user/project/service_desk/configure.md: in the existing prerequisite that already covers incoming email setup, name the observable symptom: the settings section is not displayed without it.

Related issues

Related to #603182 (quick action conditions are invisible to users; this MR is a docs-side fix for one command's condition).

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• 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.

---

AI-Generated Content Disclosure: This MR was prepared with assistance from Claude Code. The content has been reviewed for correctness, verified against the application source, and checked against the documentation style guide.