Make external links to open in a new tab

### Where should the content be added?

https://conda-forge.org/docs/user/contributing.html

### What should be added?

- I noticed that all external links in the documentation do not open in a new tab, I think it should be best to open all external links like the GitHub doc link in a new tab so as to keep the user on the conda-forge doc and aid easy navigation.
- Also I noticed some terms are outdated like that of the branch name as ``master`` instead of ``main``

### Additional information