Add quickstart guide for nodejs SDK

[simolus3]

We should merge (and release) powersync-ja/powersync-js#516 before publishing this.

This:

1. Adds NodeJS as a card next to the other client SDKs on the overview and client introduction pages (and refactors that component into a snippet).
2. Adds a basic page for the NodeJS SDK that explains how to get started (it shows adding the dependency, defining the schema, instantiating the database and connecting) as well as how to run queries (by linking to the web SDK for a complete reference and showing a simple snippet with the important operations).

Also adds placeholder sections for .NET