Improve docs in src/node/app

[nhooyr]

https://github.com/cdr/code-server/tree/docs/src/node/app#app

@code-asher

Can we describe why we had to implement VS Code remote/web and what that means? And how that's different from our patch.

Maybe we should rename src/node/app to src/node/vscode?