Add new r/docs/... examples

[moul]

• r/docs/hello - A simple introductory example.
• r/docs/adder - An interactive example to update a number with transactions.
• r/docs/source - View realm source code.
• r/docs/avl_pager - Paginate through AVL tree items.
• r/docs/img_embed - Demonstrates how to embed an image.
• markdown examples
• JSON examples
• txlink examples
• composition examples
• safe object examples
• use r/sys/users
• user r/sys/users and r/demo/profile
• use a factory to generate something
• register something against a registry
• some famous solidity patterns
• ...