Reference new template to ingest a new stack

.gitlab/issue_templates/ingest-new-stack.md

+/title Ingest <stack>
+
+/milestone %<milestone>
+
+/label ~"activity::deployment"
+
+Summary:
+
+<Replace this by a small summary or context for the new stack>
+
+Plan:
+- [ ] [Implement lister](https://docs.softwareheritage.org/devel/swh-lister/tutorial.html)
+- [ ] [Implement loader](https://docs.softwareheritage.org/devel/swh-loader-core/package-loader-tutorial.html)
+- [ ] docker: [Run lister](https://docs.softwareheritage.org/devel/swh-lister/run_a_new_lister.html)
+- [ ] docker: Run loader
+- [ ] Document lister
+- [ ] [Document loader](https://docs.softwareheritage.org/devel/swh-loader-core/package-loader-specifications.html)
+- [ ] Deploy on staging
+- [ ] Call for public review
+- [ ] Deploy on production