Update vapi docs with deepgram keyword info

[sahilsuman933]

Description

• Aligned Deepgram keywords and keyterm documentation with the API schema and Deepgram's latest guidance.
• Clarified the distinction between keywords (single words with intensifiers) and keyterm (multi-word phrases).
• Updated the example API request to correctly demonstrate both keywords and keyterm usage.
• Expanded best practices and added direct links to Deepgram's official documentation for further reference.

Testing Steps

• Run the app locally using fern docs dev or navigate to preview deployment
• Ensure that the changed pages and code snippets work

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