changes the RDoc title of the API

Nowadays the documentation of Ruby on Rails is more than the API.
encapsulates API generation in Rails::API::Task
moves RDOC_MAIN.rdoc to railties

The goal of these refactors is to have the same code generating
edge and doc:rails APIs, and the rails gem is not available in
LOAD_PATHS because it has no lib directory.
unifies API generation
rdoc is no longer a gem dependency, sdoc becomes a runtime dependency
registers the new doc:rails behaviour in the changelog