Documentation on NimMain and Memory Management

[Prince213]

Documentation on NimMain is scattered in different sections. Also, NimMain's relationship with memory management options are not clear in the documentation.

It would be nice if documentation on these topics can be improved:
NimMain, nimrtl, FFI and memory management.

[planetis-m]

It is documented in https://nim-lang.github.io/Nim/backends.html#interfacing

[Prince213]

It is documented in https://nim-lang.github.io/Nim/backends.html#interfacing

Things like NimMain aren't well documented on that page. There is some documentation, but not much.