feat: add issue and contributing templates

## Description

<!-- What needs to be done and why. -->

## Scope

<!-- What this issue covers, one item per line. -->

- [ ] 

## Deliverables

<!-- What this issue produces. -->

- [ ] 

## Acceptance criteria

<!-- Conditions that must hold before the issue is closed. -->

- [ ] 

# Contributing

Thank you for your interest in contributing to OpenOP.

Read the [OpenOP contribution guide](https://oop.etsi.org/documentation/community/contribute/)

before you open an issue or a merge request. It explains who can contribute, how

to report issues, and how code and documentation changes are reviewed.

When you open an issue in this project, choose the matching issue template and

fill in every section.