Verify development workflow docs

Description

Write Verify development docs describing workflows / patterns used to improve reliablity / resiliency.

Related to gitlab-com/www-gitlab-com!90446 (comment 679824669)

Tasks

• Add a specific Definition of Done section to the Pipeline Execution handbook page with some workflows, processes, patterns and code reviews
• Update the section for Pipeline Execution with the link to the DoD for PE once added

/cc @cheryl.li