Add manual vulnerability creation page

What does this MR do and why?

Related to #301003 (closed)

This is the first batch of a commit series that will introduce the manually vulnerability creation page. This commit prepares the page structure and creates a route in the backend. The route is only accessible through the URL and is not yet linked to the UI. Therefore, there is no need to develop this behind a feature flag.

Screenshots or screen recordings

See the original design here: #204818[m01_Manual-result_first-run-experience.png]

design	implementation

How to set up and validate locally

1. Navigate to /-/security/vulnerabilities/new through the URL (should be a project scope)
2. The page should render with a title and description

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #301003 (closed)