Configurable merge train pipeline limits (4/4)

What does this MR do and why?

This is MR 4 of 4 in a stacked series that adds configurable merge train pipeline limits, addressing gitlab-org/gitlab#374188.

MR	Branch	Base	Content
1	374188-1-add-max-pipelines-per-merge-train-plan-limit	master	Database migrations (!228370 (merged))
2	374188-2-backend-logic-and-api	374188-1-add-max-pipelines-per-merge-train-plan-limit	Backend logic, REST API, GraphQL, tests (!228371 (merged))
3	374188-3-admin-and-project-ui	374188-2-backend-logic-and-api	Admin UI, Project UI, validations (!228380 (merged))
4 (this MR)	374188-4-documentation	master	Documentation

This MR adds documentation for the configurable merge train pipeline limit feature across all relevant documentation pages.

Files to review in this MR

This MR is based on master and contains only documentation changes.

• doc/ci/pipelines/merge_trains.md - Replace hardcoded "twenty" with configurable limit description, add admin and project-level instructions
• doc/administration/instance_limits.md - New section "Merge train parallel pipeline limit"
• doc/administration/settings/continuous_integration.md - Add new limit to CI/CD Limits list
• doc/api/plan_limits.md - Add max_pipelines_per_merge_train to GET response, PUT params table, and PUT response
• doc/api/projects.md - Add max_pipelines_per_merge_train to 4 GET response tables and 2 PUT parameter tables

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

• Merge trains user documentation updated
• Instance limits administration documentation added
• Admin CI/CD settings documentation updated
• Plan Limits API documentation updated
• Projects API documentation updated
• All documentation passes markdownlint, vale, and docs-link-check

Related issues

Part of gitlab-org/gitlab#374188 (parent epic: gitlab-org&2219)