Add shebang recipe interpolation docs

[ianthetechie]

A recipe like the following is valid as far as I can tell:

greet name:
   #!/usr/bin/env sh
   echo "Hello, {{name}}!"

However, the Shebang Recipes doc does not mention this so it's difficult for users to discern what's possible.

I would happily open a PR but I see that the README now says contributions are not accepted :/

[ianthetechie]

Here's a diff in case you change you mind. Happy to open a PR. ianthetechie@5c0a464

[bluefing]

The docs do say

Expressions and Substitutions

Various operators and function calls are supported in expressions, which may be used in assignments, default recipe arguments, and inside recipe body {{…}} substitutions.

https://just.systems/man/en/expressions-and-substitutions.html#expressions-and-substitutions.

It could be argued that a recipe body is a recipe body whether its i linewise, shebang or [script] body.

Not thst I'm arguing :) just saying that shebang is not more special than [script] in this case.