Add a simple guide / sample for Commands

[octref]

Writer: @mjbvz
Reviewer: @octref / @jrieken

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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

[jrieken]

don't forget

• menus,
• keybindings
• when clauses
• arguments
• the fact that anyone can call your commands with any arguments

[octref]

@jrieken I think you understand what the emphasis of this document should be more than I do, so adding you as a reviewer.

[mjbvz]

Fixed by e87428b

[octref]

@mjbvz Can we keep this issue open for review feedbacks?
@jrieken Please take a look.

[octref]

LGTM. @jrieken if you have more feedback please open new issues.