This documentation gets deployed to: https://docs.coronalabs.com/
Engine source can be found here
Documentation lives in markdown/ directory. Note that it does not use GitHub flavored markdown, but rather its own flavor.
Plugin documentation lives in submodule. Mind it when checking out. Submodule documentation is auto-generated and not intended to be edited manually.
To build documentation yourself from bin directory run lua build.lua. Make sure Lua has LFS (Lua File System) accessible.