Document adding/updating files during the build

[mehulkar]

Couple more cookbook topics that the guides don't cover (afaik)

1. Adding files during the build process (especially if the contents need to be fetched async)
2. Changing the contents of a file during build (useful for static JSON in public/ for example

[locks]

Where is the cookbook section of the guides?

[mehulkar]

There is none at the moment! The topic of cookbook came up in a DM with @jenweber