Move services subkeys into the YAML reference

Problem to solve

While reviewing the docs at https://docs.gitlab.com/ee/ci/services/index.html#available-settings-for-services, I realized that all of these are probably not exactly "settings", but subkeys for the services keyword: https://docs.gitlab.com/ee/ci/yaml/#services

We can see that services:pull_policy is documented as a subkey: https://docs.gitlab.com/ee/ci/yaml/#servicepull_policy (though it has a typo, service should have an s).

So we should verify that:

• These are in the CI/CD schema and described properly (https://gitlab.com/gitlab-org/gitlab/-/blob/2ae502b1f297537eaabdfe48bf29e90c4e2a06ee/app/assets/javascripts/editor/schema/ci.json#L518-596).
• These keywords are added to the keyword reference (https://docs.gitlab.com/ee/ci/yaml/#services):
  • services:name
  • services:entrypoint
  • services:command
  • services:alias
  • services:variables

Other links/references