docs: extract forc chapters into its own Forc reference

Currently the Forc chapters within the Sway book is a bit out of place - it's not really teaching the Sway language but teaching the reader how to use Forc, manage the Forc.toml, etc. This is like the [Rust book](https://doc.rust-lang.org/stable/book/) <> [Cargo book](https://doc.rust-lang.org/cargo/).

I think it'd be nicer/neater to have a separate Forc reference. We can extract the Forc chapters out of the Sway book and start with that, but as we add more documentation to that we can take inspiration from the writing style of Cargo book and the [Sway reference](https://fuellabs.github.io/sway/master/reference/).

Start by locating the Forc chapters within the Sway book and reviewing how the existing Sway reference is organized. Extract the Forc-focused material into a separate Forc reference, with the resulting documentation structure and links working as the completion criteria.

Written by the indexing model from the issue text.