Allow scheduling releases

[milanmlft]

It is now possible to set release dates for episodes. Episodes with a release date occuring in the future will not be rendered.

The release dates in the lessons config.yaml should be formatted like this:

...
releases:
    "2024-05-01": introduction.Rmd
    "2025-05-01":
        - not-yet-released.Rmd
        - not-yet-released-2.Rmd

with the dates in YYYY-MM-DD format.

You can see an example of this here: https://milanmlft.github.io/sandpaper-test-releases/

It is possible to have the same release date for multiple episodes. If there is no release date for an episode, it will always be rendered.

Fixes #83

To do

• Check that this works for https://github.com/milanmlft/sandpaper-test-releases