Document holiday categories

[arkid15r]

Also, what is the definition and criteria for UNOFFICIAL (this seems to be missing from the documentation as well)? Would National Fruitcake Toss Day be included in this library or not? If so, why?

Originally posted by @mborsetti in #1669 (comment)

We should add a section explaining each holiday category purpose with holiday examples (ideally multiple entities holidays would be included).

[arkid15r]

@mborsetti please feel free to add/correct the description in case I'm missing something.

[PPsyrius]

We can probably start with PUBLIC ones, which are officially recognized holidays that applies to everyone in a particular jurisdiction.

WORKDAY on the other hand are observed by that jurisdiction's government but no holidays are given. Example for this would be United States' "Cinco de Mayo" due to its US Congress concurrent resolution status or Thailand's "Teachers' Day" as per gazetted back in 1956.

UNOFFICIAL would be those which are commonly celebrated by citizens of a particular jurisdiction but with no official observance - example of this would be Halloweens and Valentine's Day in the United States.