Enable runnable (WASM) examples for datetime

[AllenJB]

Related to #4799

Including fixing markup to make sure output replacement works correctly.

Additional changes I made:

• Normalize examples to omit unnecessary trailing ?> (PER coding style, also I see new users using this too often and suspect the manual is partly to blame)
• Moved an example from date() to mktime() (it primarily uses mktime(), and there's alternative (IMO better) ways to do that);
• Added missing output examples where appropriate;
• Modified microtime() examples to more reliably output something other than "0";
• Corrected plurality of "above example(s)";
• Misc. other minor typos and updates for deprecations

The first example on strftime() isn't working. I suspect this is due to the WASM environment not having locales or something. Since the function is deprecated I decided not to both investigating too deeply and just marked it as not runnable.