Add dotcom theme launch checklist issue template (#7025)

.github/ISSUE_TEMPLATE/theme_launch.yml

@@ -0,0 +1,15 @@
+name: DotCom theme launch checklist
+description: Checklist used to keep track of theme launches on DotCom.
+title: '[DotCom Launch]: <theme name>'
+body:
+    - type: markdown
+      attributes:
+          value: |
+              ### DotCom launch checklist
+              - [ ] Test the theme on dotCom
+              - [ ] Create demo site
+              - [ ] Create theme showcase
+              - [ ] Theme launch post
+              - [ ] Launch theme
+
+              Remember to add a label to this issue, or creating it if it doesn't exist. Format: [Theme] _<theme name>_