Reduce cross references in merge train docs

Problem to solve

Merge Trains is a great feature and I wanted to use it, so I went to the docs: https://docs.gitlab.com/ee/ci/merge_request_pipelines/pipelines_for_merged_results/merge_trains/#requirements-and-limitations

• The docs only list the Squash limitation, not Fast Forward or Semi Linear
• That requirements section links me to https://docs.gitlab.com/ee/ci/merge_request_pipelines/pipelines_for_merged_results/index.html#pipelines-for-merged-results-premium as a prerequiste, but it's not clear why the two features are related
• Having clicked the link, the anchor is dead.
• The setup instructions show me a checkbox to enable the feature and then a warning below to setup something else... https://docs.gitlab.com/ee/ci/merge_request_pipelines/pipelines_for_merged_results/index.html#enabling-pipelines-for-merged-results

Why are these docs spread over so many pages?

Further details

Proposal

Links / references