docs-utils auto-reset

at the moment:

- if you update `utils.json` you must remember to run `make reset` to actually pull in the new version of the docs-utils files if you already have a build cache

proposal:

- when you update `utils.json`, make will reset the build cache itself and fetch the appropriate docs-utils files

implementation notes:

this should be possible if `utils.json` file is a prereq for the build cache dir. however, any update to the file will trigger a reset. but this is probably fine, given that file's singular purpose