Add Code Examples to existing Documentation

[smcvb]

Take overall look on current existing documentation and add code sample where ever things are explained.

Start example giving both Configuration API and Spring (Boot) configuration (if applicable).

Per default use the GiftCard domain in the code samples.

[abuijze]

For code samples in different styles (Spring Boot vs Configuration API), we should use the codetabs plugin for gitbook: https://github.com/GitbookIO/plugin-codetabs