Move Pipelines and Builds definitions to their own page

doc/ci/pipelines.md

+# Introduction to pipelines and builds
+
+>**Note:**
+Introduced in GitLab 8.8.
+
+## 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 individual runs of [jobs]. Not to be confused with a `build` job or
+`build` stage.
+
+## Defining pipelines
+
+Pipelines are defined in `.gitlab-ci.yml` by specifying [jobs] that run in
+[stages].
+
+See full [documentation](yaml/README.md#jobs).
+
+## Seeing pipeline status
+
+You can find the current and historical pipeline runs under **Pipelines** for your
+project.
+
+## Seeing build status
+
+Clicking on a pipeline will show the builds that were run for that pipeline.
+
+[builds]: #builds
+[jobs]: yaml/README.md#jobs
+[stages]: yaml/README.md#stages
+[runners]: runners/README.md