Write documentation for the "add manual vulnerability" feature

NOTE: This issue is blocked by:

• #301003 (closed)
• #301005 (closed)

The following functionality will be released in a future iteration & has been removed from the scope of this issue: #333620

Now that the "add manual vulnerability" feature is complete, we need to enable the feature flag that was added in #301005 (closed) to show the Submit Vulnerability button, and add documentation for the feature.

Implementation details

• Set the feature flag to on by default.
• Add documentation for the feature.

Documentation guidance from design issue

This will be net new functionality so we need to clearly document the new feature including:

• The different methods for accessing and creating (in GitLab and via API)
• General workflow, specifically what happens to a newly created vulnerability
• Suggestions on best practices such as create vulnerabilities in the proper project, provide as much information during creation as possible, etc.