add ability to schedule announcements

[andrewpbray]

Allow the user to add not just docs but also announcements to appear on the course website according to the _schedule.yml. It would produce a metadata file that would be merged with _quarto.yml to create this type of functionality:

website:
  announcement: 
    icon: info-circle
    dismissable: true
    content: "Stat 20 is hiring! If you're interested in working as a tutor or reader in the spring, fill out the [New Staff Application](https://forms.gle/c5CBb1P8KTmmD5pu5). Applications received by Oct. 29th will get priority."
    type: primary
    position: below-navbar