Add FAQ link to Issue template (#14020)

.github/issue_template.md

@@ -5,7 +5,8 @@
     2. Please ask questions or configuration/deploy problems on our Discord
        server (https://discord.gg/gitea) or forum (https://discourse.gitea.io).
     3. Please take a moment to check that your issue doesn't already exist.
-    4. Please give all relevant information below for bug reports, because 
+    4. Make sure it's not mentioned in the FAQ (https://docs.gitea.io/en-us/faq)
+    5. Please give all relevant information below for bug reports, because
        incomplete details will be handled as an invalid report.
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