Move Pipelines and Builds definitions to their own page

doc/ci/README.md

 ## GitLab CI Documentation
 
-### CI/CD User documentation
+### CI User documentation
 
 - [Get started with GitLab CI](quick_start/README.md)
-- [CI/CD Definitions](definitions/README.md)
 - [CI examples for various languages](examples/README.md)
 - [Learn how to enable or disable GitLab CI](enable_or_disable_ci.md)
+- [Pipelines](pipelines.md)
 - [Environments and deployments](environments.md)
 - [Learn how `.gitlab-ci.yml` works](yaml/README.md)
 - [Configure a Runner, the application that runs your builds](runners/README.md)

doc/ci/definitions/README.md

-## CI/CD Definitions
-
-### Pipelines
-
-A pipeline is a group of [builds] that get executed in [stages] (batches). All of
-the builds in a stage are executed in parallel (if there are enough concurrent
-[runners]), and if they all succeed, the pipeline moves on to the next stage. If
-one of the builds fails, the next stage is not (usually) executed.
-
-### Builds
-
-Builds are runs of [jobs]. Not to be confused with a `build` stage.
-
-### Jobs
-
-Jobs are the basic work unit of CI/CD. Jobs are used to create [builds], which are
-then picked up by [Runners] and executed within the environment of the Runner.
-Each job is run independently from each other.
-
-### Runners
-
-A runner is an isolated (virtual) machine that picks up builds through the
-coordinator API of GitLab CI. A runner can be specific to a certain project or
-serve any project in GitLab CI. A runner that serves all projects is called a
-shared runner.
-
-### Stages
-
-Stages allow [jobs] to be grouped into parallel and sequential [builds]. Builds
-of the same stage are executed in parallel and builds of the next stage are run
-after the jobs from the previous stage complete successfully. Stages allow for
-flexible multi-stage [pipelines]. By default [pipelines] have `build`, `test`
-and `deploy` stages, but these can be defined in `.gitlab-ci.yml`. If a job
-doesn't specify a stage, the job is assigned to the test stage.
-
-### Environments
-
-Environments are places where code gets deployed, such as staging or production.
-CI/CD [Pipelines] usually have one or more deploy stages with [jobs] that do
-[deployments] to an environment.
-
-### Deployments
-
-Deployments are created when [jobs] deploy versions of code to [environments].
-
-[pipelines]: #pipelines
-[builds]: #builds
-[runners]: #runners
-[jobs]: #jobs
-[stages]: #stages
-[environments]: #environments
-[deployments]: #deployments

doc/ci/environments.md

@@ -52,7 +52,7 @@ Clicking on an environment will show the history of deployments.
 Only deploys that happen after your `.gitlab-ci.yml` is properly configured will
 show up in the environments and deployments lists.
 
-[Pipelines]: quick_start/README.md
+[Pipelines]: pipelines.md
 [jobs]: yaml/README.md#jobs
 [environments]: #environments
 [deployments]: #deployments