docs: cover possibiity of strict interface

[lread]

Currently

Babashka cli describes part of its philosophy in its intro as:

Open world assumption: passing extra arguments does not break and arguments can be re-used in multiple contexts.

This sentiment is good for some users...

...but I feel it should also include the fact that bb cli can provide a stricter interface where extra arguments, for example, are considered errors.

So...

I'm happy (as discussed on Slack) to take a stab at slightly reworking this part of the intro and adding a new short section that talks about how to move from a lax to a stricter cli. This would talk about :required, :restrict, and :validate (referencing existing sections).