Break down and remove `nuzzle.generator` namespace

[stelcodes]

nuzzle.generator is a grab bag of functions that don't have much in common except that they generate data.

The transform-config function is misplaced. As part of the config loading process, it should go in nuzzle.config. create-tag-index and create-group-index are only used by transform-config and should also be moved to nuzzle.config as well. generate-page-list is only used by the generate-*-index functions and can be incorporated into them by refactoring the functions into reduce-kv operations. The generate-*-index functions themselves can be moved to nuzzle.publish and nuzzle.ring respectively.

[stelcodes]

Also nuzzle.ring should be renamed to nuzzle.server but that might be a separate issue.