Add more examples to documentation

[mfvanek]

Hello there,
I'm a newbie in Kora. Past two weeks I've faced with many issue during development of my REST-api app.
It would be great and helpful to extend current Kora documentation with more examples.

1. Example of POST method with body here
   Like that

    @Json
    @HttpRoute(method = HttpMethod.POST, path = "/books")
    public BookEntity createBook(@Json BookCreationRequest book) {
        LOGGER.info("createBook is called with {}", book);
        return bookRepository.addNewBook(book);
    }

2. Example of how to customise returned http code of GET/POST endpoint (404, 201 etc.) here
3. Example of repository that creates new row in database and returns id here
   I mean something like that

    @Query("insert into books(title, author, published_date) values(:title, :author, :publishedDate) returning id")
    int addNewBook(String title, String author, LocalDate publishedDate);