Add issue template about decommissioning node(s)

This can be iterated upon to improve.

Refs. swh/infra/sysadm-environment#4740

.gitlab/issue_templates/decommission-nodes.md

+/title Decommission node(s) \
+/milestone %<milestone>
+
+Inventory: <link to the inventory page>
+
+Summary:
+Decommission a node is a multiple process steps which impacts repositories and inventory
+references.
+
+Tasks:
+- [ ] Update puppet swh-site node references (if any)
+- [ ] Actually decomission the node (+ commit/push if within sysadm-provisioning)
+- [ ] Update inventory references
+  - [ ] delete or deactivate the node(s)
+  - [ ] Deprecate or delete the ip addresses
+- [ ] Decommission node reference(s) in puppet master `root@pergamon# swh-puppet-master-decommission <node_0> ...<node_n>`
+- [ ] <other actions if needed>