Migrate to using multiple issue templates.

The existing issue template tries to cover everything, and as a result it is a bit overwhelming. Using multiple issue templates trims the noise when you know what kind of issue you're submitting.

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Let us know if you found a bug or encountered an error.
+---
+
+# Bug report
+<!--
+  Thanks for submitting a bug report!
+  
+  Please fill out this template to provide us with the information we need
+  to get the bug fixed as quickly as possible :)
+  
+  If you don't use the template, we may reject the issue.
+-->
+
+**Short description**
+<!-- What is the bug? What happens? -->
+
+When I join the arena, X doesn't work.
+
+
+**Reproduction steps**
+<!-- How can we reproduce the bug? -->
+
+1. Join an arena with /ma join
+2. Hit a sign to pick a class
+3. Error appears in console
+
+
+**Details**
+<!-- Server info and pastebin with stacktrace -->
+
+* **MobArena version**: 0.1.2
+* **Server version**: Spigot 3.4.5
+* **Stacktrace** (if applicable): https://pastebin.com/gB5qbcnr
+
+
+**Additional info**
+<!-- Is there anything else we need to know? -->

.github/ISSUE_TEMPLATE/documentation.md

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+---
+name: Documentation
+about: Help us improve the documentation.
+---
+
+# Documentation
+<!--
+  Thanks for helping improve the documentation!
+  
+  Please fill out this template to provide us with the information we need
+  to work out the kinks in the documentation :)
+  
+  If you don't use the template, we may reject the issue.
+-->
+
+**Description**
+<!-- What needs to be documented or improved? -->
+
+The documentation for X is confusing, making Y difficult.
+
+
+**Additional info**
+<!-- Is there anything else we need to know? -->

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest a new feature or an improvement for an existing one.
+---
+
+# Feature request
+<!--
+  Thanks for sharing your idea!
+  
+  Please fill out this template to provide us with the information we need
+  to start working on your idea as quickly as possible :)
+  
+  If you don't use the template, we may reject the issue.
+-->
+
+**Short description**
+<!-- Pitch your idea in a few sentences. -->
+
+We should have a way to do X with Y.
+
+
+**Implementation details**
+<!-- How would this work? Is it customizable? What does it affect? -->
+
+A per-arena setting could make Z configurable.
+
+
+**Additional info**
+<!-- Is there anything else we need to know? -->