Reorganize existing Secure features documentation

Problem to solve

Documentation for Secure features is suboptimal and not easy to find/consume. For example, the reports are only mentioned in the merge request documentation, but it also mentions the pipeline view report.

We should also add use cases and complete flows how to use those features.

Proposal

Reconsider documentation for Secure features in a holistic way and make it a first class entry in our documentation website.

What does success look like, and how can we measure that?

People will find documentation useful :)

Todo list

1. Migrating existing docs
   1. doc/README.md#secure > doc/user/security/index.md
   2. https://docs.gitlab.com/ee/user/project/merge_requests/#security-reports-ultimate -> link to new location doc/user/security/index.md
   3. (move content) https://docs.gitlab.com/ee/user/project/merge_requests/#interacting-with-security-reports-ultimate > doc/user/security/index.md
   4. Container scanning doc/user/security/container_scanning/index.md
      1. (redirect and merge content) https://docs.gitlab.com/ee/user/project/merge_requests/container_scanning.html
      2. (redirect and merge content) https://docs.gitlab.com/ee/ci/examples/container_scanning.html
   5. Dependency scanning doc/user/security/dependency_scanning/index.md
      1. (redirect and merge content) https://docs.gitlab.com/ee/user/project/merge_requests/dependency_scanning.html
      2. (redirect and merge content) https://docs.gitlab.com/ee/ci/examples/dependency_scanning.html
   6. DAST doc/user/security/dast/index.md
      1. (redirect and merge content) https://docs.gitlab.com/ee/user/project/merge_requests/dast.html
      2. (redirect and merge content) https://docs.gitlab.com/ee/ci/examples/dast.html
   7. License Management doc/user/security/license_management/index.md
      1. (redirect and merge content) https://docs.gitlab.com/ee/user/project/merge_requests/license_management.html
      2. (redirect and merge content) https://docs.gitlab.com/ee/ci/examples/license_management.html
   8. SAST doc/user/security/sast/index.md
      1. (redirect and merge content) https://docs.gitlab.com/ee/user/project/merge_requests/sast.html
      2. (redirect and merge content) https://docs.gitlab.com/ee/ci/examples/sast.html
   9. Security dashboard doc/user/security/security_dashboard/index.md
      1. (redirect and merge content) https://docs.gitlab.com/ee/user/project/security_dashboard.html
      2. (redirect and merge content) https://docs.gitlab.com/ee/user/group/security_dashboard/index.html
2. Find existing occurrences (both in docs and the app) and change the location

Before

├── ci
|    ├── examples
|       ├── code_climate.md
|       ├── code_quality.md
|       ├── container_scanning.md
|       ├── dast.md
|       ├── dependency_scanning.md
|       ├── license_management.md
|       ├── sast.md
|       ├── sast_docker.md
├── user
|   ├── group
|       ├── security_dashboard.md
|   ├── project
|       ├── merge_requests
|       │   ├── code_quality.md
|       │   ├── code_quality_diff.md
|       │   ├── container_scanning.md
|       │   ├── dast.md
|       │   ├── dependency_scanning.md
|       │   ├── license_management.md
|       │   ├── sast.md
|       │   ├── sast_docker.md
|       ├── security_dashboard.md

After

├── user
|   ├── security
|   |    ├── container_scanning/index.md
|   |    ├── dast/index.md
|   |    ├── dependency_scanning/index.md
|   |    ├── index.md
|   |    ├── license_management/index.md
|   |    ├── sast/index.md
|   |    └── security_dashboard/index.md