Improve CI/CD config for SSE project template for speed, reliability, DRYness, and documentation

Problem to solve

Improve speed, reliability, DRYness, and documentation for the CI config in the SSE project template

Links

• Spike in "Unfiltered" sandbox project, and related discussion/review: gl-static-site-editor/sse-unfiltered#1 (comment 429138006)
• Project template: https://gitlab.com/gitlab-org/project-templates/static-site-editor-middleman
• Caching:
  • Documentation: https://docs.gitlab.com/ee/ci/caching/#caching-ruby-dependencies
  • https://docs.gitlab.com/ee/ci/yaml/#cachekey
  • Example: https://gitlab.com/gitlab-org/gitlab-ui/-/blob/4714e73555268104dcba5f4a8f9725d9d355e364/.gitlab-ci.yml#L69-75 Closes #4 (closed)