add instructions for installing dependencies using UV

[arab0v]

Adding missing step in contributing.md file. using uv run X is not required when you already included step where you activate the virtual environment. I didn't remove it because I thought you might have kept it for a specific purpose.

[arab0v]

@igorbenav
I don't have access to edit the PR to add you as a reviewer and add labels

[igorbenav]

using uv run X is not required when you already included step where you activate the virtual environment.

Yeah, but I put the activate virtual environment kind of as an optional step (I always just use uv run). Thanks for the PR!