[Docs] Add installation troubleshooting for npm v7+ and build steps

[PratapShashwat]

I encountered 404/500 errors during installation due to missing build steps and dependency conflicts.

This PR adds a 'Troubleshooting' section to README.md to help new contributors.

Includes specific fixes for:

1. Dependency Conflicts: Added instructions for `npm install --legacy-peer-deps` (required for npm v7+).
2. Missing Frontend Assets: Added step to run `npm run build` inside the webapp folder (fixes 500/FileNotFound errors).
3. Jupyter Connection: Added commands to manually enable server/notebook extensions (fixes 404 errors).