Use the word "service" consistently for external status check

What does this MR do?

The current documentation tend to use the word "External Status Check" for everything.

• The check is a one time action, which result into a state "passed", "pending", "failed"
• This check is calculated by an external tool called "service" or "external service"

In the UI of GitLab the column is already called "service name"

In the current documentation page, the notion of "service" or "external service" is already present:

This MR tries to make the usage of the word "service" more consistent through the documentation pages

In addition, the REST API page is re-ordered to make easier to understand:

• First comes CRUD (create, read, update, delete) operations for the service.
• Then comes the endpoint about posting an external status endpoint or retrying one.

All the content is preserved, this is just a re-order

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.

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.