No documentation for bank holiday API

[KludgeKML]

We don't have any documentation for the bank holiday APi (https://www.gov.uk/bank-holidays.json), nor the clocks API (https://www.gov.uk/when-do-the-clocks-change.json).

This was noted in #5574, especially as api.gov.uk points to the calendar.md file (the internal documentation), which is confusing for users.

api.gov.uk link: https://www.api.gov.uk/gds/bank-holidays/#bank-holidays

• Add documentation for bank holidays and clocks change APIs
• Contact api.gov.uk to get them to change the link