Improve Log4j 2 example

[vy]

• Switch from deprecated JSON Layout to its successor JSON Template Layout
• Use variable names in capitals (the recommended convention by Log4j)
• Explain configuration using AsciiDoc callouts
• Use ECS-compliant layout (which avoids the need to define a Logstash filter)

This work is delivered as a part of apache/logging-log4j2#2510.

[vy]

@jsvd, is there anything else I can provide further to get this merged and/or speed things up?