Draft: Simplify pipeline configuration

This is a big simplification of the pipeline configuration, following on from !3567 (merged).

With the addition of variables for each pipeline "name", we can make the rules easier to read by using those pipeline names. On top of this, after a review of the current rules, I found:

• rules_scheduled is not used at all, so we can remove it.
• rules_scheduled_manual is only used for docker images, so we should name it as such.
• Other docker image jobs for testing changes to Dockerfiles all use identical rules, so we should use a SSOT rule.
• Use CI_PIPELINE_SOURCE as a standard alternative, when possible.
• The if: '$CI_COMMIT_BRANCH =~ /docs-preview/' is no longer needed anywhere.
• .rules_global_nav_test: was identical to .rules_site_tests, so we can remove it and use the site tests rules.
• The eslint-sast SAST job is deprecated (and was actually removed in 15.4) so we shouldn't keep it in the pipeline: gitlab#352554 (closed)

Related issues

• Related to #1324 (closed)
• Related to gitlab#389812 (closed)