More tech. info needed on message concept pages

[alexander-yevsyukov]

Consider the Rejection concept page.

It says a bit about what is a Rejection, and that it's a Protobuf message. But the text leaves the reader with the question: “What do I do to lean more about it? How do I use them in my code?”

We need to have either a brief tech info right in the concept page, or a link to a guide of declaring and using rejections (and other messages).

[meizabel]

Right now on Rejection page we have a link to the Defining Rejections Guide. Please see:
For detailed instructions on creating rejection messages, refer to Defining Rejections Guide.

[alexander-yevsyukov]

We need similar pages for Command and Event concepts.