fix: Fix broken links for specific guides section in adding_a_new_resource.md (#3174)

#### Summary
I took a look on the `adding_a_new_resource.md` in `contributing` folder and found out that links in specific guides section are broken and resulted in 404. 

After go around the repo, I think the correct link should be as following for `AWS`, `Azure` and `GCP`.
- AWS -> `../plugins/source/aws/CONTRIBUTING.md`
- Azure -> `../plugins/source/azure/CONTRIBUTING.md`
- GCP -> `../plugins/source/azure/CONTRIBUTING.md`

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