Add API documentation stubs for Vulnerabilities API changes

What does this MR do?

• Adds a stub for the Vulnerability Findings API according to the implementation plan for First-class vulnerabilities MVC. Vulnerability Findings API is what the current Vulnerabilities API will become.
• Adds a stub for new Vulnerabilities API that will provide access to the First-class Vulnerabilities
• Adds a stub for the list of the API resources where the Vulnerability Findings API and Vulnerabilities API will be mentioned properly

NOTE: the issue to actualize these documentation stubs: #34213 (closed)

Related issues

• First-class vulnerabilities backend MVC implementation
• Get single Vulnerability API call

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• [-] (not applicable because these docs are stubs) Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

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 and that you merge the equivalent EE MR before the CE MR if both exist.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.

/label documentation