Document types of pipelines

What does this MR do?

Partially addresses https://gitlab.com/gitlab-org/gitlab-ce/issues/27755. Adds a new section to the documentation for Types of Pipelines.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

• Partially addresses #27755 (closed).
• Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/22631.