Give code examples in Messaging concepts

[alexander-yevsyukov]

We need to name annotations and method conventions for the message handlers such as “Command Handler” or “Event Subscriber”.

There should be also a small code snippet which displays how a method which handles this kind of messages looks like.

[meizabel]

Use step 3 from https://spine.io/ index page.
Update the following names:
getBuilder() -> builder().
newBuilder() -> vBuilder().

[armiol]

@alexander-yevsyukov you have already modified the Concepts page to include the code samples. What we have left is to describe the conventions.