README.md: better distinction between single pipeline definition versus multiple instances [docs]

What does this MR do?

While a single .gitlab-ci.yml can spawn many, concurrent pipeline instances, it can define a single pipeline type. Increase the use of singular pipeline to avoid users wasting time searching how to define more than one pipeline type. Steer users towards the needs: keyword which the only way to break free from the strict ordering of stages.

Related issues

Longer story in the comments of gitlab-foss issue #19471, start from October 2019.

Author's checklist

• [ X ] Follow the Documentation Guidelines and Style Guide.
• [ N/A ] If applicable, update the permissions table.
• [ N/A ] Link docs to and from the higher-level index page, plus other related docs where helpful.
• [ I wish - no permission to add labels? ] Apply the documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.