Create a Pull Request Template for contributions to the OCaml Cookbook and update CONTRIBUTING.md

[sabine]

To simplify people self-reviewing their submissions, we need a checklist in a PR Template to communicate the guidelines for new cookbook recipes, especially on whether a new task is admissible.

At the same time, these guidelines need to be added to CONTRIBUTING.md.

Guidelines from https://discuss.ocaml.org/t/feedback-help-wanted-upcoming-ocaml-org-cookbook-feature/14127/37?u=sabine

[labdhiongithub7]

@sabine I would like to do this task.

[sabine]

@labdhiongithub7 thanks for offering, would be glad if you can prepare a PR template!

[labdhiongithub7]

Hi @sabine, could you please review #2907 and let me know if any updates are needed?
Thanks!!