Gather all the CI/CD Configuration concepts together

Problem to solve

At the moment, .gitlab-ci.yml is given top-billing as Configuration in CI/CD land, but it diminishes other configuration. For example:

• Project configuration.
• AutoDevOps configuration.

In Global Nav, it's given the name Configuration too.

Proposal

Create a new CI/CD configuration page, that has all the CI/CD configuration aggregated and linked out to.

Demote .gitlab-ci.yml to being just one type of configuration.

Other links/references

• Existing CI content: https://gitlab.com/gitlab-org/gitlab-ce/issues/58326
• Refactoring main page: https://gitlab.com/gitlab-org/gitlab-ce/issues/58323.