Provide official logos

Fixes https://github.com/ocaml/ocaml.org/issues/44
Update index.md

Add link to EPTCS
OCaml 2014: Update index.md

Add link to EPTCS
Merge pull request #457 from garrigue/patch-1

Update index.md
Create modules.zh.md
Serve the logos from ocaml.org

Fixes https://github.com/ocaml/ocaml-logo/issues/2
Add link to the Logo page in the footer
Create null_pointers_asserts_and_warnings.zh.md