Request: Add CC BY-NC 4.0 and CC BY-NC-SA 4.0 to licenses

I see that CC BY and CC BY-SA are in `_licenses/`, but the non-commercial versions are nowhere to be found on this repo. For the files see [CC BY-NC-SA 4.0](https://raw.githubusercontent.com/vitorsr/cc/refs/heads/master/CC-BY-NC-SA-4.0.txt) and [CC BY-NC 4.0](https://raw.githubusercontent.com/vitorsr/cc/refs/heads/master/CC-BY-NC-4.0.txt)

Start by reviewing the existing CC BY and CC BY-SA files in `_licenses/` to follow the repository's format. Add the CC BY-NC 4.0 and CC BY-NC-SA 4.0 license files using the linked source texts. Done means both licenses are present in `_licenses/` and match the referenced texts.

Written by the indexing model from the issue text.