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.
This will allow us to programatically position .rst snippets *anywhere*
which will for once and for all remove the horrid title level mismatch bugs.
We require this in order to allow people to re-shuffle the spec without
having to adjust the spec itself (e.g. 2 targets with different levels of
nesting).
We're well beyond the point now where a simple `cat` of .rst files to "build"
the spec is practical. We may want to slice and dice the spec in different
ways to address various cross-cutting concerns. To this end, there is now a
'targets' file which contains the "build targets" for the spec, which contains
the sorting order for the .rst files. For now, we just have a single
target: 'main'.
