Bring building documentation in line

While looking at how to build log4j, i noticed several discrepancies; I looked at the branch release-2.x on github and on https://logging.apache.org/log4j/2.x/build.html

The website seems outdated, atleast the files it lists don't exist (anymore?). Luckily the instructions in BUILDING.md work. If it is possible to somehow just display the content of the BUILDING.md in that section of the website (or just link to it) it should simplify keeping the instructions up to date.

I think it would be good if README.md under "Building From Source" simply referred to BUILDING.md, similar to the "Contributing" section. With both changes there is then only one document to update when building procedures change.