Add transactions example for couchbase

[mikereiche]

Since transactions are only available starting with spring-data-couchbase 5.0.0 (spring-boot-starter-parent 3.0.0) this module will need to have a different parent than the other spring data couchbase examples.

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>3.0.0-SNAPSHOT</version>
    <relativePath/> <!-- lookup parent from repository --> 
</parent>

[christophstrobl]

Thanks @mikereiche - merged to the boot-3 branch via 1f50e84afff0d50c47ccbfcc95396086332e1d64