Document the AST/items_tree/template API

[Elv13]

(this issue split of a discussion in a ticket regarding removing the sandbox and supporting modules)

Currently, the API used by the template isn't fully documented. There is a lot of things in there, but you have to read the ldoc code to learn about them. Given how mature ldoc is, I think it's time to commit to this API and document it. Along with #333 and #335, this is necessary to implement more maintainable modules to extend ldoc.