Add link to pipeline type documentation in the merge request page

From comment https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14664#note_204981791:

This merge request https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14664 updates the text of the pipeline widget on a merge request's page.

This new text simplifies the current implementation (which has an unwieldy number of v-if in the Vue template) and also better indicates the pipeline's purpose.

Now that the mention the pipeline's purpose (Merge train pipeline, Merged result pipeline, etc), it would be ideal to allow users to have an exit point in the interface where they could learn more about those different pipeline types.

Suggestion

Add a link/helper to the UI in the merge request page that let users quickly learn more (documentation) about the pipeline type that is running.

Implementation guide

In the component app/assets/javascripts/vue_merge_request_widget/components/mr_widget_pipeline.vue specifically the element with the data-testid pipeline-details-container add the help popover component '~/vue_shared/components/help_popover.vue'. Then add the related messages outlined in the design The popover text should be dynamic based on the event_type_name