docs: Document the new file cache
docs: Document the new file cache

See #5404

gohugoio/hugo,mdhender/hugo,anthonyfok/hugo,fedelibre/hugo,anthonyfok/hugo,fedelibre/hugo,fedelibre/hugo,mdhender/hugo,gohugoio/hugo,mdhender/hugo,mdhender/hugo,gohugoio/hugo,fedelibre/hugo,anthonyfok/hugo