Add more readable datetime literal examples

[alexjpwalker]

Goal

We've done a pass over our date & time literal examples to keep them as intuitive as possible

Implementation

Split date & datetime examples into two blocks - one block for literals without the leading + symbol to denote a non-four-digit year - this is the default that everyone should expect & be used to - and we separate out the examples with a leading + into a separate clearly indicated block

For example:

Example date literals
2024-03-30
1920-09-21
#!test[read]
#{{
match
  let $x = 2024-03-30;
  let $y = 1920-09-21;
#}}

Example date literals (non-4-digit year)
+20000-01-01
+900-09-21
#!test[read]
#{{
match
  let $x = +20000-01-01;
  let $y = +900-09-21;
#}}