Document third party integration for Security Reports

Problem to solve

We need to create dedicated documentation for 3rd party integrations into our Security Reports.

Further details

We currently expose the JSON format of each report type on their respective documentation:

Container Scanning and DAST to be done with the corresponding migrations: (DAST migration, Container Scanning migration)

Proposal

Create a new page ~~or section~~ dedicated to integration, to be added to the GitLab Secure documentation.

This documentation should present and explain the JSON properties that are already listed SAST and Dependency Scanning. In particular, it should:

present the mandatory properties, and explain why they are needed
explain how identifiers and locations work, and provide examples

It should cover the shared report format as well as what's specific to SAST, DAST, Dependency Scanning, and Container Scanning.

Every type of scanner must be documented. Not available for this scanner is a valid documentation. For each Not available scanner type, create a backlog issue to add / update documentation once specified blocker is complete.

If we decide to rename the report format, use ~~Secure Stage Common Report Format SSCRF~~ To be discussed.

We should also document the ~~standards~~ conventions the CI job definition of the scanners should follow, such as:

scripts must be in the script: section, and leave before_script: and after_script: free for user customizations
(to be completed)

The documentation should explain how the job definitions are made available to users, using YAML snippet they include in their CI configuration files.

A further proposal is available in this doc: https://docs.google.com/document/d/1nlChfHQM9cei1h3NNoNnyza9Hfm79kvDLVKYc5awVQ0/edit#heading=h.fq83kn48tw2