[stephenvapiai]

Adds an "Override a saved assistant for one call" section to the Transient vs permanent configurations page.

The page showed assistantOverrides in one example but never explained it. The new section covers:

• What assistantOverrides changes and that the saved assistant is left unchanged
• Outbound and web calls, including a note that batch calls share one set of settings
• Inbound calls, where the assistant-request response returns assistantId and assistantOverrides
• Pinning a published version with assistantVersion, and how it differs from overrides
• Overrides on squad members and handoff destinations, with links to the existing squad and handoff pages
• A note that live call control can't change the model, voice, or prompt mid-call

Only fern/assistants/concepts/transient-vs-permanent-configurations.mdx is changed.