Docs: Cleaning up the merge request workflow

What does this MR do?

As per https://gitlab.com/gitlab-org/gitlab-ce/issues/51355, this MR fixes grammar, links, and generally dusts off the merge request workflow.

Ported to EE in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9249 due to failing ee_compat_check.

Related issues

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/51355

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24559

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24559), this MR afterward