Document OpenTofu Support

[osterman]

what

• Document how to use OpenTofu together with Atmos

why

• OpenTofu is a stable alternative to HashiCorp Terraform
• Closes #542

[osterman]

new tf alias can be brought up via a GH issue if we find that we want to discuss that further.

@Gowiem there might be some confusion on aliases; they can be anything you want.

Here's some suggestions by @nitrocode (they're aggressively short!)

aliases:
  h: help
  v: version
  c: completion
  da: describe affected
  dcm: describe component
  dcn: describe config
  dd: describe dependents
  ds: describe stacks
  t: terraform
  tc: terraform clean
  td: terraform deploy
  tgb: terraform generate backend
  tgbs: terraform generate backends
  tgv: terraform generate varfile
  tgvs: terraform generate varfiles
  ts: terraform shell
  tw: terraform workspace
  hf: helmfile
  hgv: helmfile generate varfile
  vc: validate component
  vs: validate stacks
  vp: vendor pull
  w: workflow
  aeuk: aws eks update-kubeconfig
  agrc: atlantis generate repo-config

Then what you can do is alias a=atmos and run any of these like this: a t for atmos terraform