Whole-file documentation

It would be nice if there was a way to document the entire justfile and include it in `--list`, which would be helpful for overviews or mentioning how commands relate. For example:

```just
##! This handles the setup and running of foo. Note that commands bar
##! and baz interact, and if you run qux you will eventually later need to
##! run quux. If you are just getting started, you probably want corge.

# First recipe
default:
just --list

# ...
```

Could render as:

```text
$ just
This handles the setup and running of foo. Note that commands bar
and baz interact, and if you run qux you will eventually later need to
run quux. If you are just getting started, you probably want corge.

Available recipes:
```