Writer: @mjbvz
Reviewer: @octref / @jrieken

Add a guide to Extension Guides / Command, and a sample at vscode-extension-samples.

• Explain 3 things: How they relate to each other, what's the use of each
  • vscode.commands.executeCommand
  • vscode.commands.registerCommand
  • contributes.commands
• Point to https://vscode-ext-docs.azurewebsites.net/api/references/commands