Update assignment color references in interviewer dashboard docs

[Copilot]

Assignment color guidance in the Interviewer Dashboard docs was stale after the UI harmonized assignment indicators to purple (#A371F7). This updates the remaining outdated references so the page matches current assignment card and Create New tab behavior.

• Scope

  • Update the Interviewer Dashboard workload guide to describe assignment-related UI elements with the current purple color.
  • Keep the change limited to assignment-specific references; interview status colors remain unchanged.

• Doc updates

  • Create New tab color: grey → purple
  • Assignment card Start button: blue → purple
  • Assignment workflow text in the “How to create a new interview” section: blue Start → purple Start

• Consistency

  • Align this older guide with the existing release notes and dashboard card documentation that already describe assignments as purple.

- interface will be grey for the **Create New** tab
+ interface will be purple for the **Create New** tab

- blue ***Start*** button on the assignment
+ purple ***Start*** button on the assignment

[radyakin]

1. Change last updated date on the page to the date of the change. Field: lastmod.
2. Image for assignments ("Create new" status/page) still shows the grey header, even though the text says "purple" after the change.
3. The new button title for creating an interview is "START NEW INTERVIEW" (purple color).