Documentation updates for 1.7.0-alpha1 (state encryption) (#1396)

Signed-off-by: Janos <86970079+janosdebugs@users.noreply.github.com>

internal/encryption/method/config.go

@@ -0,0 +1,21 @@
+// Copyright (c) The OpenTofu Authors
+// SPDX-License-Identifier: MPL-2.0
+// Copyright (c) 2023 HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+
+package method
+
+// Config describes a configuration struct for setting up an encryption Method. You should always implement this
+// interface with a struct, and you should tag the fields with HCL tags so the encryption implementation can read
+// the .tf code into it. For example:
+//
+//	type MyConfig struct {
+//	    Key string `hcl:"key"`
+//	}
+//
+//	func (m MyConfig) Build() (Method, error) { ... }
+type Config interface {
+	// Build takes the configuration and builds an encryption method.
+	// TODO this may be better changed to return hcl.Diagnostics so warnings can be issued?
+	Build() (Method, error)
+}