Document string-parsing FAQ in the string-parsing docs page

[justingrant]

In js-temporal/temporal-polyfill#133, I answered what have been common questions around Temporal's string parsing. At some point, we should probably add a FAQ to the existing docs page about the string format: https://tc39.es/proposal-temporal/docs/iso-string-ext.html. Here's some questions that could go there, most of which are answered in js-temporal/temporal-polyfill#133.

• What's the right Temporal type to use for parsing a particular string?
• Why is Temporal.PlainDateTime usually the wrong type to use for string parsing, and what type should I use instead?
• When should I use toString vs. toLocaleString when converting Temporal types to a string?
• Why is a bracketed time zone annotation required for ZonedDateTime.from?
• Why can't I parse a UTC "Z" string using Temporal.PlainXxx types?
• In the unusual case where I want to get the calendar/clock units from a UTC "Z" string, how can I do that?
• What industry standards apply to the string formats that Temporal uses?
• When and how will Temporal's string extensions be standardized?
• How do Temporal string serialization formats relate to those used by other platforms and libraries?
• Why doesn't Temporal support parsing localized formats like MM/DD/YY?

While we're there, we should probably update that page to discuss the IETF standardization work and downplay ISO because we're not planning to standardize with ISO at this point.