Re-add GitHub issues config from old docs repo

Signed-off-by: Julius Volz <julius.volz@gmail.com>

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,9 @@
+blank_issues_enabled: true
+contact_links:
+  - name: Prometheus Community Support
+    url: https://prometheus.io/community/
+    about: If you need help or support, please request help here.
+  - name: Commercial Support & Training
+    url: https://prometheus.io/support-training/
+    about: If you want commercial support or training, vendors are listed here.
+  - name: Blank issue template

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,7 @@
+<!--
+    Please sign CNCF's Developer Certificate of Origin and sign-off your commits by adding the -s / --sign-off flag to `git commit`
+
+    More information on both of those can be found at https://github.com/apps/dco
+
+    If you are proposing a new integration, exporter, or client library, please include representative sample output.
+ -->