Add more documentation and doctests to the builtins

[nekevss]

Currently, the project has been relatively relaxed as far as documentation and doctests, but these are also incredibly useful for any users looking to use temporal_rs. Furthermore, it allows us to really get a better feel for the API while also providing more tests.

My initial thought is that we should try to have doctests for the builtins below and their main methods that may have high user visibility:

• PlainDate
• PlainDateTime
• PlainTime
• ZonedDateTime
• Duration
• Instant
• PlainYearMonth
• PlainMonthDay