Where are the Docs?

I’m regularly running into the problem that the documentation available on docs.gitlab.com makes no sense to me whatsoever, as it seems to be written for the self-hosted version of GitLab.
Where can I find documentation for the use of GitLab.com functionality?

For example: I’m trying to configure a .gitlab-ci.yml in order to build a docker image from a Dockerfile. My search draws me to this page which almost immediately starts talking about all kinds of irrelevant (to me) stuff like installing Gitlab Runner and configuring users and group permissions.