feat: add state import doc

docs/vendors/terraform/state-management.md

@@ -128,6 +128,40 @@ To do this, use the following steps:
 
 ## Importing existing state file into your Terraform Stacks
 
+### On an existing stack
+
+State import allows you to import a state on top of the latest managed state.
+
+!!! warning
+
+    You should really understand what you are doing when importing a state.
+
+To be able to import a state, the 3 conditions below must be satisfied:
+
+- You must be a stack admin
+- The stack must be locked
+- The stack must not have any pending runs or tasks
+
+To import a new state you need to go to the state history tab on your stack, and then click on the button to import the state.
+
+![](../../assets/screenshots/terraform/state-management/import_state_action.png)
+
+!!! info
+
+    The maximum allowed file size for a state is **100MB**.
+
+Then choose a valid terraform state file and upload it.
+
+![](../../assets/screenshots/terraform/state-management/import_state_modal.png)
+
+Once the file is uploaded, you can click on **Import state**.
+
+The imported state will appear in the list as manually imported.
+
+![](../../assets/screenshots/terraform/state-management/import_state_list.png)
+
+### During stack creation
+
 When creating a stack, you can optionally import an existing Terraform state file so that Spacelift can manage it going
 forward.