Create the ‘!pandoc’ Tup macro

This takes the Pandoc command from ‘content/Tupfile’ and defines it as
the ‘!pandoc’ macro from a new Tuprules file.  This will make it
possible to create sub-directories in ‘content/’ that use this macro
so that I do not have to copy-paste the same command into each Tupfile
in each of those directories.
Add a Tupfile for the ‘content/BOTW/’ directory
Add article ‘BOTW: Programming in Lua 3rd Ed.’