Update the links in all docs except user and admin pages

This is not used on the current docs website
and needs to be removed for future compatibility
with Hugo.

Because we changed the way we
do the markdown.

Related to: https://gitlab.com/gitlab-org/gitlab-docs/-/issues/1690

Replacing "This is" with "Basic pipelines."
Change "Basic Pipelines" to sentence case.
Removed "easy-to-find" due to subjective wording.

Stop using top level keywords and only/except

Pipeline Insights becomes Pipeline Security, and
many categories shifted around.

Final small copy edits to finish cleaning up the page,
and also adding more examples, and improving
old examples.

Do a full CTRT rewrite on the downstream pipelines content.
Simplify language as much as possible, and separate into
task and concept sections. Reduce duplication, and make the
examples clearer.

Related epic: https://gitlab.com/groups/gitlab-com/-/epics/1940

Part 3 of combining multi-project pipelines and parent-child
pipelines together.

Adds tier badges to several CI documentation pages.

The Continuous Integratin group was renamed as
Pipeline Execution.

The previous URL had two drawbacks:

1. It didn't match what the title was about (`Assignments to DevOps
   Stages and Groups`, and the anchor was `#designated-technical-writers`).
2. It didn't account for the other sections, like development and API
   guidelines.

Part two of a ci doc cleanup

Ran yamllint on yaml codeblocks to identify
minor formatting inconsistencies

needs datatype is of array type.

Adds doc explaining the different ways to
structure pipelines, to guide users to the
features and CI keywords they need.