Update issue and pull request templates

.github/ISSUE_TEMPLATE/bug_report.md

@@ -1,36 +1,51 @@
 ---
 name: Bug report
-about: Create a report to help us improve
+about: Report unexpected or incorrect behaviour
 title: ''
 labels: bug
 assignees: ''
 
 ---
 
-**Describe the bug**
+## Describe the bug
 
-A clear and concise description of what the bug is.
+<!--
+  A clear and concise description of what the bug is.
 
-If you have a problem with a specific package or NixOS,
-you probably want to file an issue at https://github.com/NixOS/nixpkgs/issues. 
+  If you have a problem with a specific package or NixOS,
+  you probably want to file an issue at https://github.com/NixOS/nixpkgs/issues.
+-->
 
-**Steps To Reproduce**
+## Steps To Reproduce
 
 1. Go to '...'
 2. Click on '....'
 3. Scroll down to '....'
 4. See error
 
-**Expected behavior**
+## Expected behavior
 
-A clear and concise description of what you expected to happen.
+<!-- A clear and concise description of what you expected to happen. -->
 
-**`nix-env --version` output**
+## Metadata
 
-**Additional context**
+<!-- Please insert the output of running `nix-env --version` below this line -->
 
-Add any other context about the problem here.
+## Additional context
 
-**Priorities**
+<!-- Add any other context about the problem here. -->
+
+## Checklist
+
+<!-- make sure this issue is not redundant or obsolete -->
+
+- [ ] checked [latest Nix manual] \([source])
+- [ ] checked [open bug issues and pull requests] for possible duplicates
+
+[latest Nix manual]: https://nixos.org/manual/nix/unstable/
+[source]: https://github.com/NixOS/nix/tree/master/doc/manual/source
+[open bug issues and pull requests]: https://github.com/NixOS/nix/labels/bug
+
+---
 
 Add :+1: to [issues you find important](https://github.com/NixOS/nix/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc).

.github/ISSUE_TEMPLATE/feature_request.md

@@ -1,24 +1,39 @@
 ---
 name: Feature request
-about: Suggest an idea for this project
+about: Suggest a new feature
 title: ''
 labels: feature
 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 [...]
+## Is your feature request related to a problem?
 
-**Describe the solution you'd like**
-A clear and concise description of what you want to happen.
+<!-- A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] -->
 
-**Describe alternatives you've considered**
-A clear and concise description of any alternative solutions or features you've considered.
+## Proposed solution
 
-**Additional context**
-Add any other context or screenshots about the feature request here.
+<!-- A clear and concise description of what you want to happen. -->
 
-**Priorities**
+## Alternative solutions
+
+<!-- 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. -->
+
+## Checklist
+
+<!-- make sure this issue is not redundant or obsolete -->
+
+- [ ] checked [latest Nix manual] \([source])
+- [ ] checked [open feature issues and pull requests] for possible duplicates
+
+[latest Nix manual]: https://nixos.org/manual/nix/unstable/
+[source]: https://github.com/NixOS/nix/tree/master/doc/manual/source
+[open feature issues and pull requests]: https://github.com/NixOS/nix/labels/feature
+
+---
 
 Add :+1: to [issues you find important](https://github.com/NixOS/nix/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc).

.github/ISSUE_TEMPLATE/installer.md

@@ -23,14 +23,25 @@ assignees: ''
 
 <details><summary>Output</summary>
 
-```log
+<!-- paste console output inside the below code block -->
 
-<!-- paste console output here and remove this comment -->
+```log
 
 ```
 
 </details>
 
-## Priorities
+## Checklist
+
+<!-- make sure this issue is not redundant or obsolete -->
+
+- [ ] checked [latest Nix manual] \([source])
+- [ ] checked [open installer issues and pull requests] for possible duplicates
+
+[latest Nix manual]: https://nixos.org/manual/nix/unstable/
+[source]: https://github.com/NixOS/nix/tree/master/doc/manual/source
+[open installer issues and pull requests]: https://github.com/NixOS/nix/labels/installer
+
+---
 
 Add :+1: to [issues you find important](https://github.com/NixOS/nix/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc).

.github/ISSUE_TEMPLATE/missing_documentation.md

@@ -26,6 +26,6 @@ assignees: ''
 [source]: https://github.com/NixOS/nix/tree/master/doc/manual/source
 [open documentation issues and pull requests]: https://github.com/NixOS/nix/labels/documentation
 
-## Priorities
+---
 
 Add :+1: to [issues you find important](https://github.com/NixOS/nix/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc).

.github/PULL_REQUEST_TEMPLATE.md

@@ -17,10 +17,12 @@ so you understand the process and the expectations.
 
 -->
 
-# Motivation
+## Motivation
+
 <!-- Briefly explain what the change is about and why it is desirable. -->
 
-# Context
+## Context
+
 <!-- Provide context. Reference open issues if available. -->
 
 <!-- Non-trivial change: Briefly outline the implementation strategy. -->
@@ -29,7 +31,7 @@ so you understand the process and the expectations.
 
 <!-- Large change: Provide instructions to reviewers how to read the diff. -->
 
-# Priorities and Process
+---
 
 Add :+1: to [pull requests you find important](https://github.com/NixOS/nix/pulls?q=is%3Aopen+sort%3Areactions-%2B1-desc).