Add Setup Guide for JS v4

Current setup guide for JS doesn't make use of the new JS v4 programming model. We should add explicit instructions for this here: https://github.com/Azure/azure-functions-sql-extension/tree/main/docs

Start by reviewing the current JavaScript setup guide in the repository's docs directory and compare it with the JS v4 programming model. Add explicit JS v4 setup instructions there, and consider the work done when the guide covers the new model clearly.

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