Rename pipelines for merge requests/merged results in the docs

Problem to solve

Related to #350225 (closed)

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)