gha: update issue templates

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,26 @@
+---
+name: Bug report
+about: Please submit opam bug reports here.
+title: ''
+labels: ''
+assignees: ''
+
+---
+<!--
+Welcome to the opam issue tracker!
+
+The best way to help us dig on the issue is to provide informations on how the
+error occurred, outputs, opam version, and ideally a reproducible case (we know
+that it is not always the case).
+
+Before opening an issue, please check if there is one already existing, even
+in closed ones: https://github.com/ocaml/opam/issues?q=is%3Aissue+
+
+If your issue concerns a package not building, please report to
+https://github.com/ocaml/opam-repository/issues or to the package bugtracker
+unless you are confident it is an issue in the opam tool itself.
+--!>
+
+```
+<include here the output of `opam config report`, if applicable>
+```

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,9 @@
+blank_issues_enabled: false
+contact_links:
+  - name: OCaml Discuss Forum
+    url: https://discuss.ocaml.org/
+    about: This is the best place to start with questions about using opam.
+  - name: opam Package Repository
+    url: https://github.com/ocaml/opam-repository/issues
+    about: >-
+      Please report packaging issues there.

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,13 @@
+---
+name: Feature request
+about: Suggest a new feature for opam.
+title: ''
+labels: 'KIND: FEATURE WISH'
+assignees: ''
+
+---
+<!--
+Welcome to the opam issue tracker!
+
+Share with us you feature request, the need and  its origin.
+-->