Add Default Construct ID documentation

[mitchlloyd]

Issue #377

This change adds documentation for using the special "Default" construct ID. This issue mentions documenting the use of "Resource" as well, but I've since learned that using "Resource" outside of creating L2 constructs doesn't have any advantages over using "Default". For creating L2 constructs, I believe that the CDK design Guidelines are best suited for the audience doing that type of work.

This documentation includes code examples that will need to be translated into multiple languages if they stay in. I'm holding off on that work and just using Github-style code blocks while iterating on the content. I've also used hard line wraps to make editing easier, but I can remove those.

The paragraph beginning "Using "Default" as a construct ID for is a good choice..." may be controversial.x

TODO:

• Examples in all supported languages
• Remove all hard wrapping

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[ghost]

This is a good contribution and a valuable topic. I appreciate your patience; I know you contributed it a long time ago. Just wanted you to know I've had a look and will try to get to it soon.

[ghost]

This is a great contribution! It will need the code examples in the other supported languages before it can be merged. We can do this, but it'll probably be a while longer.