Refactor Pipelines Pages

Pipelines are currently documented in a few different places. See https://gitlab.com/gitlab-org/gitlab-ce/issues/58326. The intent of the existing pipelines page is also unclear (it's more than an intro, now). See feedback in: https://gitlab.com/gitlab-org/gitlab-ce/issues/58323.

Ensure any existing mentions of "multi-project pipeline graphs" simply refer to "multi-project pipelines" where applicable.
Include a better description and examples of how to configure a multi-project pipeline rather than just linking to the triggers documentation.
Include more description around multi-project or cross-project pipelines, including definitions of upstream and downstream pipelines (and making sure to use all those words so people searching for documentation are more likely to find it.

Create a one-stop shop for pipelines information, ensuring it's introduced at the appropriate time, and that the content is fleshed out.