Issue template addition: Are you using Gitea behind CloudFlare? (#14098)

* chore: are you using Gitea behind CloudFlare

since more often than not CF appears to serve stale cache and cause
troubles, I'd argue it might be helpful to ask about it in this here
issue template

* implement suggestion: change question to comment

* as per @techknowlogick's suggestion

* chore: edit comment

* implement @mrsdizzie's suggestion
* as the comment grows, rather span multiple lines
* Gitea --> gitea to match case used in the rest of the template

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

.github/issue_template.md

@@ -30,6 +30,9 @@
 <!-- In addition, if your problem relates to git commands set `RUN_MODE=dev` at the top of app.ini -->
 
 ## Description
+<!-- If using a proxy or a CDN (e.g. CloudFlare) in front of gitea, please
+     disable the proxy/CDN fully and connect to gitea directly to confirm
+     the issue still persists without those services. -->
 
 ...