docs: add QueryPlanEntry and QueryPlanEntryStep to documentation

[tswast]

Is your feature request related to a problem? Please describe.

The documentation for QueryJob.query_plan shows the return type as List[QueryPlanEntry], but QueryPlanEntry is not a clickable link like other return types (example: QueryJob.ddl_target_table has a clickable link to TableReference).

Describe the solution you'd like

• Add a reference to QueryPlanEntry to the job-related types in reference.rst.
• Verify that the query_plan property docs now render with a link.