Templates don't know at what level they will be inserted. Previously, we
hard-coded the title style which is not compatible with the build target
system. Define a set of styles which will be replaced by the gendoc script
when it encounters them:
'<' : Make this title a sub-heading
'/' : Make this title a heading at the same level
'>' : Make this title a super-heading
The build target system is now basically complete and functioning.