Provide more context on tool-specific properties

Currently most (all?) templates provide an entry for customizations similar to:

```json
// Configure tool-specific properties.
// "customizations": {},
```

But new users to Codespaces and `devcontainers.json` specifically, do not understand how to add an extension or how that looks like. It would be useful to have either a link to documentation or a sample ID for a VSCode extension that could be used (or both).

The feature of right-clicking an extension and selecting "add to devcontainer.json" is incredibly useful but hardly used because there isn't awareness.

This issue in particular is brought forward based on feedback from students while learning to use Devcontainers for the first time.