Move CODEOWNERS and ISSUE_TEMPLATE.md to .github folder

Keeping CONTRIBUTING.md at the root so that contributors actually find it.

Also changed the custom FUNDING.yml link to point to godotengine.org/donate,
which also links to PayPal and other options.

.github/FUNDING.yml

@@ -1,2 +1,2 @@
 patreon: godotengine
-custom: https://www.paypal.com/donate/?token=v-mEfr__g1BbOiStnJsgwRib_I8jVSMD_064iu7NizC0Uvm9fSiPYOs-v1VQz9HFK1K8YG
+custom: https://godotengine.org/donate