Rename pipelines for merge requests/merged results in the docs

Problem to solve

We need to find all instances of pipelines for merge requests and pipelines for merged results in the docs, and use instead: merge request pipelines and merged results pipelines.

Steps:

Update text in the docs: !79197 (merged)
Rename pipelines_for_merged_results.md to merged_results_pipelines.md and add redirect: !79310 (merged)
Update global nav: gitlab-docs!2401 (merged)

Edited Jan 27, 2022 by Marcel Amirault