docs: update v3 poll mode configuration examples

[daixihegu]

Description

• Replace the outdated useQuadraticVoting poll config property with mode in the v3.x quick start docs.
• Update v3.x poll examples to use the numeric mode values expected by the deploy config:
  • 0 for QV
  • 1 for non-QV
  • 2 for full credits voting
• Keep the duplicate poll type examples in sync with the same numeric values.

Fixes #2826.

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