Chore: Update package manager in readme for the examples

[mckn]

Descriptions

Many of our examples has documentation that reference to yarn as the package manger even if we have changed to npm as the default package manager.

Todo

• [x] App with backend
• [x] App with dashboards
• [x] Datasource basic
• [x] Datasource http backend
• [x] Datasource streaming backend websocket
• [x] Datasource streaming websocket
• [x] Panel basic
• [x] Panel datalinks
• [x] Panel plotly