Update obsolete legacy Date cookbook examples

[ptomato]

I found these 3 cookbook examples were out of date, and two of them were
actually not included in the cookbook (but still executed together with
the cookbook tests.)

The examples were written before we had ZonedDateTime and also before we
had Date.prototype.toTemporalInstant().

I've consolidated them into 2 files, "fromLegacyDate" and "toLegacyDate",
and updated the accompanying explanations.

[codecov]

Codecov Report

Merging #1930 (530fa45) into main (1bcbb69) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1930   +/-   ##
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  Coverage   95.01%   95.01%           
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  Files          19       19           
  Lines       10940    10940           
  Branches     1732     1732           
=======================================
  Hits        10395    10395           
  Misses        530      530           
  Partials       15       15           

Flag	Coverage Δ
test262	81.79% <ø> (ø)
tests	89.85% <ø> (ø)

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[justingrant]

OK, I have some opinionated comments. 😄 The TL;DR is that a bad pattern from legacy Date is confusing UTC and local-TZ values, and we want to be really careful in our own docs (including cookbook) that we don't accidentally encourage the same confusion with Temporal.

[ptomato]

@justingrant I didn't incorporate all of the suggestions literally but instead rewrote the examples, and I think I got the spirit of your comments. I'd appreciate another look if you have time.

[justingrant]

Much better! Looks great. Thanks!