disable blank issues, add online docs link

.github/ISSUE_TEMPLATE/config.yml

@@ -1,4 +1,8 @@
+blank_issues_enabled: false
 contact_links:
   - name: TinyUSB Discussion
     url: https://github.com/hathach/tinyusb/discussions
     about: If you have other questions or need help, post it here.
+  - name: TinyUSB Docs
+    url: https://docs.tinyusb.org/
+    about: Online documentation