Remove all references of mix.phoenix from guides (#2488)
Remove all references of mix.phoenix from guides (#2488)

Several documents contained `mix.phoenix` this is find all and replace.

With exception for the `testing_controllers.md` where I simply added a context. We'll need to devote more time into those guides as a whole, to include contexts.
optikfluffel/phoenix,jwarwick/phoenix,optikfluffel/phoenix,michalmuskala/phoenix,evax/phoenix,rafbgarcia/phoenix,mitchellhenke/phoenix,Joe-noh/phoenix,korobkov/phoenix,bsmr-erlang/phoenix,bsmr-erlang/phoenix,dmarkow/phoenix,nshafer/phoenix,phoenixframework/phoenix,Gazler/phoenix,pap/phoenix,Kosmas/phoenix,mobileoverlord/phoenix,korobkov/phoenix,rafbgarcia/phoenix,jwarwick/phoenix,0x00evil/phoenix,stevedomin/phoenix,Gazler/phoenix,pap/phoenix,trarbr/phoenix,0x00evil/phoenix,phoenixframework/phoenix,rafbgarcia/phoenix,phoenixframework/phoenix,michalmuskala/phoenix,nshafer/phoenix,eksperimental/phoenix,nshafer/phoenix,gjaldon/phoenix,mobileoverlord/phoenix,0x00evil/phoenix,keathley/phoenix,Kosmas/phoenix,mschae/phoenix,optikfluffel/phoenix,eksperimental/phoenix,gjaldon/phoenix,Joe-noh/phoenix,evax/phoenix,stevedomin/phoenix,dmarkow/phoenix,mitchellhenke/phoenix,mschae/phoenix,trarbr/phoenix,keathley/phoenix