Document Log4j2's log4j-spring-boot module and its spring lookup as an alternative to our Logback <springProperty> extension

[wilkinsona]

Log4j 2.14 introduced a new log4j-spring-boot module. It provides a lookup plugin so that properties in the environment can be referenced from the logging configuration. See this comment for @rgoers for some concrete examples and also its section in the Log4j lookups documentation.

I think it would be good to mention this in the docs, perhaps by moving this section up one level so that it's no longer Logback-specific and then adding Logback and Log4j2 subsections to it.

There's a circular dependency between the two projects at the moment. I think that's worth discussing to check that we're comfortable with it.