Cover more material in scopes section

[sogaiu]

This PR is an attempt to address #431.

I think the points mentioned in the issue were covered. Please let me know if I missed something.

In addition, the suggested changes include:

• Surfacing of "shadowing" a bit more in prose and in examples
• Addition of some examples
• Shortening some examples (slightly compensating for the additional text that makes the overall page longer)
• Updating some comments in some examples (e.g. IIUC, some error messages have changed since the original writing)
• Some rewording in a few places

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Some specific points regarding mutation / removal of the original text:

1. I replaced the sentence:

   Scopes are useful for organizing your bindings and they can expand your programs.

   with:

   Scopes are useful for organizing your bindings and can provide flexibility in expressing your programs.

   I hope that captured the spirit. I didn't really get what "expand" meant in the original 😅

2. I removed a portion that recommends the use of let over that of do and def for at least two reasons:

   1. The existing text recommends let and then goes on to say that using do and def is fine, so it seemed that just not making the recommendation communicates almost the same idea...and that's shorter!
   2. In practice, I think there are some Janet-using folks [1] who actually prefer to use def over let in some situations [2].

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[1] Disclosure: it's true that I'm one of those people...and you're going to have to work to convince me otherwise :)

[2] It might be of interest to eventually collect together more specifics on when to use one approach over the other, but I think this PR is too narrow to contain it :P

[pepe]

LGTM ❤️

[yohannd1]

Looks really nice as well imo! Thanks!