Add graphql howto doc to style guide

What does this MR do?

Set up instructions on how to doc a GraphQL example / use case, in our style guide

Since our current style guide covers only RESTful API docs, this MR also modifies the noted section to specify "RESTful APIs". I've also updated the one (1) link I've found to this section, from our docs.gitlab.com build. (I've checked our Handbook, and do not find links from there.)

Ideally, anyone with a little knowledge of GraphQL (TW, developer, PM) should be able to use this documentation to create new GraphQL documentation for their specific use case.

I've also added info on how to include an entry in our Table of Contents. I'm surprised that we don't already have this info, and maybe it should be in a separate MR.

This is a follow-up to !31019 (merged)

Related issues

!31019 (merged)

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have developer access or higher (for example, GitLab team members or Core Team members)
  • Apply the documentation label, plus:
    • The corresponding DevOps stage and group label, if applicable.
    • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
    • development guidelines and Documentation guidelines when changing docs under development/documentation/*.
    • development guidelines and Description templates (.gitlab/*) when creating/updating issue and MR description templates.
  • Assign the designated Technical Writer.

Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• Update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Add GitLab's version history note(s).
• Add the product tier badge.
• Add/update the feature flag section.
• If you're changing document headings, search doc/*, app/views/*, and ee/app/views/* for old headings replacing with the new ones to avoid broken anchors.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
  • Add Technical Writing and docs:: workflow label.
  • Add docs-only when the only files changed are under doc/*.
  • Add twdoing when starting work on the MR.
  • Add twfinished after approving and/or merging the MR.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.