website: Revise the "Module Sources" page
website: Revise the "Module Sources" page

It's been a while since we made any significant updates to this page.
The main theme of this revamp is to ensure that we highlight how to
provide "ambient credentials" for each of the module sources using the
standard mechanisms for each source type.

While there, I also attempted a general cleanup to highlight the main
cases and make this less like a detailed description of all of
go-getter's esoteric features, and did some general copy-editing to write
it in our usual documentation "voice".

gechr/terraform,pryorda/terraform,drebes/terraform,ewbankkit/terraform,pryorda/terraform,luis-silva/terraform,iljaskevic/terraform,donaldguy/terraform,brandontosch/terraform,iljaskevic/terraform,hashicorp/terraform,hashicorp/terraform,rnaveiras/terraform,youhong316/terraform,tommynsong/terraform,drebes/terraform,heimweh/terraform,brandontosch/terraform,heimweh/terraform,VladRassokhin/terraform,apparentlymart/terraform,VladRassokhin/terraform,ewbankkit/terraform,pdecat/terraform,heimweh/terraform,apparentlymart/terraform,apparentlymart/terraform,drebes/terraform,VladRassokhin/terraform,heimweh/terraform,nlamirault/terraform,tommynsong/terraform,pradeepbhadani/terraform,ewbankkit/terraform,mattray/terraform,tpounds/terraform,pdecat/terraform,ewbankkit/terraform,tpounds/terraform,donaldguy/terraform,kl4w/terraform,pradeepbhadani/terraform,brandontosch/terraform,luis-silva/terraform,brandontosch/terraform,nlamirault/terraform,mattray/terraform,donaldguy/terraform,gechr/terraform,hashicorp/terraform,luis-silva/terraform,iljaskevic/terraform,heimweh/terraform,youhong316/terraform,gechr/terraform,pryorda/terraform,kl4w/terraform,pdecat/terraform,youhong316/terraform,iljaskevic/terraform,mattray/terraform,tpounds/terraform,pradeepbhadani/terraform,mattray/terraform,apparentlymart/terraform,apparentlymart/terraform,brandontosch/terraform,tommynsong/terraform,kl4w/terraform,pdecat/terraform,VladRassokhin/terraform,rnaveiras/terraform,rnaveiras/terraform,tpounds/terraform,donaldguy/terraform,luis-silva/terraform,drebes/terraform,kl4w/terraform,nlamirault/terraform,youhong316/terraform