Adds docs about file and folders names used in .gitlab

What does this MR do?

We're implementing features that depend on file and folder name under .gitlab/. This MR proposes to document existing features so we don't colide names when adding new features.

I've mapped the following usage of this folder:

• Custom Dashboards: .gitlab/dahboards/.
• Issue Templates: .gitlab/issue_templates/.
• Merge Request Templates: .gitlab/merge_request_templates/.
• Kubernetes Agents: .gitlab/agents/.
• CODEOWNERS: .gitlab/CODEOWNERS.
• Route Maps: .gitlab/route-map.yml.
• Auto DevOps Helm Values: .gitlab/auto-deploy-values.yaml.
• GitLab managed apps CI/CD: .gitlab/managed-apps/config.yaml.
• Insights: .gitlab/insights.yml.
• Service Desk Templates: .gitlab/service_desk_templates/.
• Web IDE: .gitlab/.gitlab-webide.yml.

Screenshots

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