Update calendar publishing instructions
Update calendar publishing instructions

- Clarify which machine to run the rake task on
- Describe how to run rake task on production, before describing how to run it locally (more likely to need the former than the latter)
- Be explicit about which pages get updated via the task
- Remove general comment about how search indexing works.

Trello: https://trello.com/c/cX3nmA5o/2766-add-bank-holidays-and-clock-change-dates-for-2023
alphagov/frontend,alphagov/frontend,alphagov/frontend,alphagov/frontend