GitHub: move default template to bug_report.md, add config

With this commit we prepare for adding multiple issue templates to
GitHub that will be shown when the user clicks on the "New Issue" button
on GitHub.
The config.yml will also add a bunch of useful links to the page that
shows the different issue templates.
An example of how this template selection page looks like can be found
here: https://github.com/lightninglabs/lightning-terminal/issues/new/choose

.github/ISSUE_TEMPLATE/bug_report.md

@@ -1,3 +1,11 @@
+---
+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.

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,11 @@
+blank_issues_enabled: false
+contact_links:
+  - name: Documentation for lnd and lightning-terminal
+    url: https://docs.lightning.engineering/
+    about: Please make sure the documentation cannot answer your question first.
+  - name: Lightning 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.