Update issue templates

.github/ISSUE_TEMPLATE/bug_report.md

+---
+name: Bug report
+about: Create a report to help us improve. If you need help setting up your simulation
+  with Arbor, please start a new topic in Discussions.
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+Steps to reproduce the behavior:
+1. '...'
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Screenshots**
+If applicable, add screenshots to help explain your problem.
+
+**System (please complete the following information):**
+ - OS: [e.g. Ubuntu]
+ - Version of relevant packages [e.g. Arbor 0.3, Conda Python 3.7]
+
+**Additional context**
+Add any other context about the problem here.