Refactor DatePicker Formats section

[rolfsmeds]

• Add a new page-level tab "Date Formats" to the DatePicker page.
• Move the contents of the Date Format section there.
• Write a brief intro section that summarizes
  • How the default format is decided
  • That in Flow you can set a Locale and/or specify custom date formats
  • That in Lit & React you can define a custom function
  • That for 2-digit year formats there's the reference date (with links to the corresponding section on the page)