Update contrib and PR template markdowns

[miguelgfierro]

Description

I've noticed that in the PR template, when we link ../CONTRIBUTING.md, the link works: https://github.com/recommenders-team/recommenders/blob/main/.github/PULL_REQUEST_TEMPLATE.md

However, in the PRs, the link takes you to the wrong URL: https://github.com/recommenders-team/recommenders/CONTRIBUTING.md

I believe we need to put in the template CONTRIBUTING.md. We will be able to check it when we merge to main.

Related Issues

References

Checklist:

• I have followed the contribution guidelines and code style for this project.
• I have added tests covering my contributions.
• I have updated the documentation accordingly.
• This PR is being made to staging branch and not to main branch.