[Maintenance:] Add contributing guide

[ByteOtter]

What does this PR change?

Add contributing guide to Documentation

Tick the applicable box:

• [ ] Add new feature

• [ ] UI improvement

• [ ] Changed routing

• [ ] Security changes

• [ ] Testsuite
  

• [x] General Maintenance

UI changes

provide description here or remove all unapplicable lines below

• No changes

  • [x] DONE

Documentation

• No documentation needed
  

• [x] DONE