Add docs on how to register toolkit web components in JS/TS

[hawkticehurst]

Feature/component description

Based on this issue, we should do a better job of documenting/highlighting how to register toolkit web components from within a JavaScript or TypeScript file.

Like so:

import { 
  provideVSCodeDesignSystem, 
  vsCodeButton, 
  vsCodeCheckbox,
  // Import other component functions –– always in the form `vsCode{ComponentName}`
} from "@vscode/webview-ui-toolkit"

provideVSCodeDesignSystem()
  .register(
    vsCodeButton(), 
    vsCodeCheckbox(),
    // Register other component functions here
  );

Use case

Toolkit users may not always want to use the toolkit via the bundled toolkit.js or toolkit.min.js files, so we should better document how to import and register individual components.

[hawkticehurst]

Reference docs we can use to model our own documentation: https://www.fast.design/docs/components/getting-started