docs: update garbage collection document
docs: update garbage collection document

- remove_built_image is now an exit plugin
- mention squash plugin parameters

maxamillion/atomic-reactor,DBuildService/atomic-reactor,jarodwilson/atomic-reactor,vrutkovs/atomic-reactor,DBuildService/atomic-reactor,vrutkovs/atomic-reactor,fr34k8/atomic-reactor,jarodwilson/atomic-reactor,fr34k8/atomic-reactor,projectatomic/atomic-reactor,jpopelka/atomic-reactor,jpopelka/atomic-reactor,projectatomic/atomic-reactor,maxamillion/atomic-reactor