.github: combine issue templates, put issue/pr template text in a comment

Author: classabbyamp

.github/ISSUE_TEMPLATE/bug-report.md

@@ -0,0 +1,7 @@
+<!--
+Before opening an issue, please be sure to check what content is suitable for
+the Handbook: https://docs.voidlinux.org/about/about-this-handbook.html
+
+We prioritise issues about information specific to Void, and try to avoid
+duplicating information already available elsewhere (e.g. on the Arch wiki).
+-->

.github/ISSUE_TEMPLATE/feature-request.md

@@ -1,3 +1,5 @@
+<!--
 Before submitting a pull request, please read [CONTRIBUTING](https://github.com/void-linux/void-docs/blob/master/CONTRIBUTING.md); pull requests that do not meet the criteria described there will not be merged. Note that this repository's CONTRIBUTING contains information specific to this repository, and is not the same as CONTRIBUTING for the void-packages repository.
 
 We prioritise pull requests involving information specific to Void over those involving information applicable to Linux in general.
+-->