Enhance documentation #374

The following links point to existing Issues which contain information not currently described in the docs or points to non documented topics:

• How to use with @aws-cdk/aws-lambda-nodejs #363
• update has "upsert" functionality #360
• DAX support #305
• Question: Docs/Change Log? #254
• Documentation: How to increase the logging level #302
• Documentation: .index must be called before .wherePartitionKey #301
• Local configuration and single table #341

Also we should think about a path forward to have the docs included in this repo to be able to make code changes and the required doc changes in one shot, contained in one PR to ensure we don't miss the doc updates.