Document getenv template function
Document getenv template function

Add `getenv` to the template function documentation.

MarkDBlackwell/hugo,mitchchn/hugo,ifraixedes/hugo,abourget/hugo,gohugoio/hugo,ffrizzo/hugo,ryanclarke/hugo,mayah/hugo,ryanclarke/hugo,vbatoufflet/hugo,halostatue/hugo,mdhender/hugo,mitchchn/hugo,derekperkins/hugo,tischda/hugo,mdhender/hugo,fhergomez/hugo,coderzh/hugo,jygastaud/hugo,fedelibre/hugo,digitalcraftsman/hugo,fedelibre/hugo,tjyang/hugo,badoet/hugo,derekperkins/hugo,gohugoio/hugo,vbatoufflet/hugo,sprig/hugo,kujohn/hugo,bogem/hugo,anthonyfok/hugo,tjyang/hugo,anthonyfok/hugo,Zenithar/hugo,soundrussian/hugo,Zenithar/hugo,mayah/hugo,halostatue/hugo,melo/hugo,anthonyfok/hugo,soundrussian/hugo,bogem/hugo,tubo028/hugo,digitalcraftsman/hugo,badoet/hugo,melo/hugo,kujohn/hugo,tatsushid/hugo,bramp/hugo,MarkDBlackwell/hugo,mdhender/hugo,gohugoio/hugo,bramp/hugo,ifraixedes/hugo,ffrizzo/hugo,fedelibre/hugo,abourget/hugo,tubo028/hugo,jygastaud/hugo,tatsushid/hugo,tischda/hugo,fhergomez/hugo,mdhender/hugo,coderzh/hugo,fedelibre/hugo,sprig/hugo