document Final Review page: Add a "Final Review started on..." line at the top

[ajeanmahoney]

Below the page heading. For example:

RFC-to-be 9888 (draft-ietf-stir-servprovider-oob) (In Final Review)
Final Review started 2025-10-21

¶

[rudimatz]

@ajeanmahoney how exactly would you define "Final Review started on..."?
When the "final_review_editor" assignment was created? (=an editor as assigned as final_review_editor, even if the state of the assignment is currently just assigned and not yet in_progress)

[ajeanmahoney]

When the "final_review_editor" is assigned

[rudimatz]

once PR ietf-tools/purple#1313 is merged, this info is available in API client