chore: use quarto freeze option

[JosiahParry]

I am trying to review the ISC call for proposals on the r-consortium website. However, quarto is trying to render every single post on the website. This results in an error with renv and some missing packages when building certain pages.

To make it easier to contribute (and prevent borking non-backwards compatible blog posts) I suggest using the freeze feature from quarto and only re-rendering posts / pages only when the content has changed.

The current approach makes it difficult to contribute.

[jcasman]

@JosiahParry I agree this is an issue. We going to look into fixing this. I will respond back here when we've made progress.