Add static site editor documentation

What does this MR do?

This adds documentation for the Static Site Editor feature.

To view it locally you can navigate to: http://localhost:3000/help/user/project/static_site_editor/index.md

Related issues

#212557 (closed)

Author's checklist

• Add first draft documentation.
• Technical writer & PM review.
• Follow the Documentation Guidelines and Style Guide.
• Apply the documentation label.
• Add Global Nav item to the docs project (gitlab-docs!788 (merged)).

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.

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.