docs: migrate realtime server implementation

[KianYang-Lee]

This commit moves the realtime server for testing WebSockets and Server Sent Events (SSE) in htmx that was originally in the htmx repository to the current repository.

Added a new package.json file to install ws dependency, since originally it relies on the project root package.json. Also updated the loading of htmx.js file to use CDN instead, and corrected the relative paths for the ws.js and sse.js files.

This should be viewed in pair with PR https://github.com/bigskysoftware/htmx/pull/2955

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[Telroshan]

Thanks !