docs: Document symdiff
docs: Document symdiff

Closes https://github.com/gohugoio/hugoDocs/issues/659

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