add docs on managing state with digger (#1974)

Author: motatoes

docs/ce/howto/managing-state.mdx

@@ -0,0 +1,82 @@
+---
+title: "Managing state"
+---
+
+In this secion we cover approaces to manage state with digger's automation.
+Currently digger does not interfere with terraform's state management and we leave it up to the user
+to manage the state on their own accounts. This is because we realised that most of the users come to us with state
+already managed somewhere, usually an S3 bucket in their own account, and we didn't want to add an extra migration step whilst moving to digger.
+
+With that said, we do facilitate state management configuration within digger in order to make it easy for teams to manage it on their own account.
+This guide will cover an approach to facilitate this state management.
+
+The example repo for this is here: https://github.com/diggerhq/states-test
+
+In this example we have the followign directory structure:
+
+```
+dev/
+    main.tf
+    tf_backend.tfbackend
+staging/
+    main.tf
+    tf_backend.tfbackend
+prod/
+    main.tf
+    tf_backend.tfbackend
+```
+
+within each main.tf root module we define a backend block:
+```
+terraform {
+  backend "s3" {
+
+  }
+}
+```
+
+We ommit the backend state name, key and region on purpose since it is defined in the file tf_backend.tfbackend within the same directory:
+
+```
+bucket="digger-state-test"
+key="/dev/terraform.tfstate"
+region="us-east-1"
+```
+
+This is done in staging/ and prod/ as well. We consider it as a convention for all the root modules.
+
+With that in place, we can configure digger to pass this additional configuration while running terraform as follows:
+
+```
+projects:
+  - name: "dev"
+    dir: "dev"
+  - name: "staging"
+    dir: "staging"
+  - name: "prod"
+    dir: "prod"
+
+
+workflows:
+  default:
+    workflow_configuration:
+      on_pull_request_pushed: ["digger plan"]
+      on_pull_request_closed: ["digger unlock"]
+      on_commit_to_default: ["digger unlock"]
+
+    plan:
+      steps:
+      - init:
+        extra_args: ["-backend-config=tf_backend.tfbackend" ]
+      - plan:
+    apply:
+      steps:
+      - init:
+        extra_args: ["-backend-config=tf_backend.tfbackend" ]
+      - apply:
+```
+
+The key part here is that we override the default workflow and pass extra arguments of `-backend-config=tf_backend.tfbackend` to the `plan` and `apply` steps.
+In this way it is easy to add additional states simply by adding a record for them in digger.yml. Once a PR is created and applied we will end up with a bucket that has three state files:
+
+![](/images/configuration/state-bucket-example.png)

docs/images/configuration/state-bucket-example.png

@@ -77,6 +77,7 @@
     {
       "group": "How To",
       "pages": [
+        "ce/howto/managing-state",
         "ce/howto/specify-terraform-version",
         "ce/howto/apply-on-merge",
         "ce/howto/apply-requirements",