WIP: Reorganize the Secure features under the same path

What does this MR do?

Reorganize the Secure features under the same path. This will result in a cleaner view of the docs.

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.md
|   |    ├── dast.md
|   |    ├── dependency_scanning.md
|   |    ├── index.md
|   |    ├── license_management.md
|   |    ├── sast.md
|   |    └── security_dashboard.md

Related issues

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/7189

Author's checklist

1. Apply the correct labels and milestone
2. 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.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.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.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.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/dast.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.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
3. Find existing occurrences (both in docs and the app) and change the location

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards