Moved QLDB code examples into qldb directory; added create-ledger code example (#485)

* Moved QLDB code examples into qldb directory; added create-ledger code example

* Removed some leftover qldb files

* Renamed QLDB helloword.rs as qldb-helloworld.rs

* Updated create-ledger code example based on feedback

* Added readme with descriptions of all three QLDB code examples; slightly modified the description of the ledger in the hello world code example

* Update QLDB readme

Small language tweak suggesting the high level driver

Co-authored-by: Russell Cohen <rcoh@amazon.com>