Add PR template and links to contributor guidelines

[Joonalai]

Summary

This PR applies OSGeo Suomi contributor guidelines to this repository.

Related issues

How this was tested

Contributor checklist

• I have read the CONTRIBUTING guidelines.
• I have reviewed and understand every change in this pull request, and I take responsibility for it as the author. This applies whether the changes were written by hand or with the assistance of AI tools.
• If AI tools were used to generate a substantial part of this contribution, I have disclosed it in the PR description above or via an Assisted-by: commit trailer.
• My contribution is licensed under the same license as this repository, and I have the right to submit it.
• I have run the project's tests locally where applicable.