Document the process/command API

[mattwparas]

See here for the module

It is a little opaque - the functions that return Result return a struct to rust that is Ok or Err, which makes it non obvious how to use. On top of that, some of the functions are builders, and some act via mutation. First is to probably add some documentation to these functions, and then after that we should implement a module in the cogs directory that wraps up some of the more low level functions into something a bit more idiomatic.

For example, capturing std out into a string is a little non obvious:

(define (with-stdout-piped command)
  (set-piped-stdout! command)
  command)

(~> (command "git" (list "check-ignore" "target"))
    (with-stdout-piped)
    (spawn-process)
    (Ok->value)
    (wait->stdout)
    (Ok->value))