chore: add ISSUE_TEMPLATE (#264)

Signed-off-by: Ryota Sakamoto <[email protected]>

Author: ryota-sakamoto

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Reproducible bug report 
+about: Create a reproducible bug report. Not for support requests.
+labels: 'bug'
+---
+## Summary 
+
+What happened/what you expected to happen?
+
+## Diagnostics
+
+What Kubernetes provider are you using? 
+
+What version of Argo CD and Argo CD Notifications are you running? 
+
+```
+# Paste the logs from the notification controller
+
+# Logs for the entire controller:
+kubectl logs -n argocd deployment/argocd-notifications-controller
+```
+
+---
+<!-- Issue Author: Don't delete this message to encourage other users to support your issue! -->
+**Message from the maintainers**:
+
+Impacted by this bug? Give it a 👍. We prioritise the issues with the most 👍.

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+
+contact_links:
+  - name: Have you read the docs?
+    url: https://argocd-notifications.readthedocs.io/
+    about: Much help can be found in the docs
+  - name: Ask a question
+    url: https://github.com/argoproj-labs/argocd-notifications/discussions/new
+    about: Ask a question or start a discussion about notifications
+  - name: Chat on Slack
+    url: https://argoproj.github.io/community/join-slack
+    about: Maybe chatting with the community can help

.github/ISSUE_TEMPLATE/enhancement_proposal.md

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+---
+name: Enhancement proposal
+about: Propose an enhancement for this project
+labels: 'enhancement'
+---
+# Summary
+
+What change needs making?
+
+# Use Cases
+
+When would you use this?
+
+---
+<!-- Issue Author: Don't delete this message to encourage other users to support your issue! -->
+**Message from the maintainers**:
+
+Impacted by this bug? Give it a 👍. We prioritise the issues with the most 👍.