Add CODE_OF_CONDUCT.md, CONTRIBUTING.md, and SECURITY.md.

These "community health files" provide important information about
the project and will be displayed prominently on the PostgreSQL
GitHub mirror.  For now, they just point to the website, but we may
want to expand on the content in the future.

Reviewed-by: Peter Eisentraut, Alvaro Herrera, Tom Lane
Discussion: https://postgr.es/m/20240417023609.GA3228660%40nathanxps13

.github/CODE_OF_CONDUCT.md

@@ -0,0 +1,2 @@
+The PostgreSQL code of conduct can be found at
+<https://www.postgresql.org/about/policies/coc/>.

.github/CONTRIBUTING.md

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+For information about contributing to PostgreSQL, see
+<https://www.postgresql.org/developer/>.

.github/SECURITY.md

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+For information about reporting security issues, see
+<https://www.postgresql.org/support/security/>.