github: add issue template

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a bug report. Please use the discussions section for general or troubleshooting questions.
+title: '[bug]: '
+labels: ["bug", "needs triage"]
+assignees: ''
+---
+
+### Background
+
+Describe your issue here.
+
+### Your environment
+
+* version of `btcd`
+* which operating system (`uname -a` on *Nix)
+* any other relevant environment details
+
+### Steps to reproduce
+
+Tell us how to reproduce this issue. Please provide stacktraces and links to code in question.
+
+### Expected behaviour
+
+Tell us what should happen
+
+### Actual behaviour
+
+Tell us what happens instead

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: Discussions
+    url: https://github.com/btcsuite/btcd/discussions
+    about: For general or troubleshooting questions or if you're not sure what issue type to pick.
+  - name: Community Slack
+    url: https://lightning.engineering/slack.html
+    about: Please ask and answer questions here.
+  - name: Security issue disclosure policy
+    url: https://github.com/lightningnetwork/lnd#security
+    about: Please refer to this document when reporting security related issues.

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest a new feature for `btcd`.
+title: '[feature]: '
+labels: enhancement
+assignees: ''
+---
+
+**Is your feature request related to a problem? Please describe.**
+<!-- A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] -->
+
+**Describe the solution you'd like**
+<!-- A clear and concise description of what you want to happen. -->
+
+**Describe alternatives you've considered**
+<!-- A clear and concise description of any alternative solutions or features you've considered. -->
+
+**Additional context**
+<!-- Add any other context or screenshots about the feature request here. -->