Introduce the attribute #[doc(...)] for doc comments

[gfngfn]

#[doc(```
  `map f [v_1, …, v_n]` applies function `f` to each `v_i` in the given order,
  and builds the list [f v_1, …, f v_n] with the results produced by `f`.
```)]
val map<$a, $b> : fun(fun($a) -> $b, list<$a>) -> list<$b>

[gfngfn]

Although they have not been perfect yet, documents generated by DocumentGenerator came to be somewhat satisfactory:

source: https://github.com/gfngfn/sesterl_stdlib/blob/2765531d386c78bf6e40e319951e1b3efb3221af/src/Stdlib.sest