website: v0.12 upgrade guide revisions preparing for final release
website: v0.12 upgrade guide revisions preparing for final release

Our original upgrade guide was drafted while some things were still in
flux and not all of our upgrade tooling was in place yet.

This redraft now attempts to be more specific and direct, showing exact
commands to run and (where relevant) exact error messages that Terraform
might return.

I also took this opportunity for some general copy-editing, though we'll
probably want to do one more pass of that alone (without changing any
content at the same time) before final release.

This new content presumes the existence of a Terraform v0.11.14 release,
which isn't published yet at the time of writing but should be published
before v0.12.0 final, once we've done final verification and review of
the upgrade path including it.

nlamirault/terraform,drebes/terraform,luis-silva/terraform,luis-silva/terraform,brandontosch/terraform,rnaveiras/terraform,apparentlymart/terraform,apparentlymart/terraform,gechr/terraform,nlamirault/terraform,brandontosch/terraform,heimweh/terraform,brandontosch/terraform,heimweh/terraform,hashicorp/terraform,rnaveiras/terraform,ewbankkit/terraform,VladRassokhin/terraform,drebes/terraform,pdecat/terraform,pradeepbhadani/terraform,hashicorp/terraform,pradeepbhadani/terraform,tommynsong/terraform,brandontosch/terraform,pryorda/terraform,apparentlymart/terraform,pdecat/terraform,VladRassokhin/terraform,apparentlymart/terraform,heimweh/terraform,tommynsong/terraform,heimweh/terraform,pradeepbhadani/terraform,drebes/terraform,ewbankkit/terraform,pryorda/terraform,heimweh/terraform,VladRassokhin/terraform,hashicorp/terraform,ewbankkit/terraform,ewbankkit/terraform,apparentlymart/terraform,pdecat/terraform,drebes/terraform,gechr/terraform,pdecat/terraform,gechr/terraform,luis-silva/terraform,tommynsong/terraform,VladRassokhin/terraform,nlamirault/terraform,brandontosch/terraform,pryorda/terraform,rnaveiras/terraform,luis-silva/terraform