doc(tfstated): import documentation file from the forgejo wiki

doc/What-is-TfStated.md

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+# What is TfStated
+
+TfStated is an [OpenTofu/Terraform states HTTP
+backend](https://developer.hashicorp.com/terraform/language/backend/http).
+
+OpenTofu/Terraform state files are extremely sensitive: they describe the
+complete state of one's infrastructure including secrets and other sensitive
+data. TfStated's primary mission is to safely and securely manage these state
+files. This entails:
+
+- Storing the state files fully encrypted.
+- Controlling whose people or what systems have access to which state files.
+- Keeping an audit log of state files access and modifications.
+- Storing state files version history.
+- Allowing to rollback to previous state files versions.
+- Manage state file locks.