Pipeline Graph Structural Update: Default Flag to True

What does this MR do?

• Defaults the :graphql_pipeline_details feature flag to true. It's currently out to 100% on dot-com.
• Updates documentation as discussed in #276949 (comment 488111087)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides

Related to #276949 (closed)