Add explainer for LocalDateTime

[ptomato]

I intend to eventually adapt this into the documentation for
LocalDateTime, but given the draft status of the API this seems more
appropriate.

[codecov]

Codecov Report

Merging #816 into main will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #816   +/-   ##
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  Coverage   92.26%   92.26%           
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  Files          17       17           
  Lines        4796     4796           
  Branches      751      751           
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  Hits         4425     4425           
  Misses        369      369           
  Partials        2        2           

Flag	Coverage Δ
#test262	55.91% <ø> (ø)
#tests	87.10% <ø> (ø)

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[justingrant]

This is great. I noticed a few missing things:

• The new startOfDay property (see Easier "midnight" ? #737) is now checked into DO NOT MERGE: Proof-of-concept for ZonedDateTime (Instant + TimeZone + Calendar) #700. It should probably be in this content.
• The isTimeZoneOffsetTransition property isn't mentioned.

[sffc]

Thanks for making my suggested changes!

[ptomato]

Added highlighting to Prism in order to fix the docs build.