Added instructions to run DAST API scans

What does this MR do?

Adds instructions to help users configure a DAST scan against an API endpoint.

This MR is WIP because:

• SOAP scans have not been verified to work
• The DAST_REQUEST_HEADERS configuration value has not been merged/released gitlab-org/security-products/dast!124 (merged).
• The latest DAST API scan implementation has not been merged/released gitlab-org/security-products/dast!122 (merged).
• The rename of the DAST host override header has not been merged/released gitlab-org/security-products/dast!123 (merged).
• The Domain validation feature has been explicitly disabled for API scans. This has not been merged/released gitlab-org/security-products/dast!125 (merged)

This in part resolves issue #10928 (closed).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team