github: Add AI usage warning to issue, doc and PR templates

- Explain to contributors that though AI use is acceptable they must
  not file unless they can understand and explain their work without AI.

Assisted-by: Viktor Szakats

Ref: https://github.com/curl/curl/discussions/21792

Closes https://github.com/curl/curl/pull/21801

.github/pull_request_template.md

@@ -0,0 +1,8 @@
+<!--
+        IMPORTANT:
+        If you cannot understand or explain your work without using
+        Artificial Intelligence (AI) then do not file here. Do not paste
+        massive AI generated explanations. We accept the use of AI as long as
+        it is digestible. Please explain your issues or improvements briefly
+        and clearly in your own human voice.
+-->