Documentation: Add "life of a CI/CD job"

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We should add a concept of "the life of a GitLab CI/CD job" to our documentation. This will be useful for understanding as a user what happens when a job is executed. Will also be useful to developers and contributors to understanding the back and forth that happens between GitLab and the runner when a job is executed. This will also help the ~Verify product team understand the UX around the speed of job execution, etc.

Proposed Diagram

graph TD
   id01[git push]
   id02[git tag]
   id03[Merge request merged]
   id04[Schedule triggered]
   id05[Merge requst created]
   id08[API trigger]
   id09[Manual pipeline run]
   id06[Event triggers pipeline creation]
   id01-->id06
   id02-->id06
   id03-->id06
   id04-->id06
   id05-->id06
   id08-->id06
   id09-->id06
   subgraph GitLab
      id07[Parse .gitlab-ci.yml]
      id06-->id07
      id10[Create stages & jobs]
      id07-->id10
      id11[Add jobs to Sidekiq queue]
      id10-->id11
   end
   subgraph Runner
      id12[Pinging for new jobs]
      id13[Picks job based on tags, priority, etc.]
      id13-->id11
   end

New doc or update?

• New documentation
• Update existing documentation

Checklists

Product Manager

• Add the correct labels
• Add the correct milestone
• Indicate the correct document/directory for this feature
• Fill the doc blurb below

Documentation blurb

• Doc title

• Feature overview/description

• Feature use cases

Developer

• Copy the doc blurb above and paste it into the doc
• Write the tutorial - explain how to use the feature
• Submit the MR using the appropriate MR description template