Recommend uv for virtual environment management

[matt-graham]

Resolves #615

• Switches uv to recommended / green and conda to good / yellow in Virtual environments page.
• Changes template README to suggest using uv to create a project specific virtual environment and changes pip commands to instead be uv pip.
• I also noticed our language around docs dependencies hadn't been updated when we switched to dependency group so this also changes that.
• I have currently not changed any of the commands in template README for running tests / building docs to use uv run so these would implicitly require project virtual environment to be activated.

[paddyroddy]

I know it goes against our "one green per tool", but I wonder if pixi should also be recommended. Many scientific projects may use dependencies that are only on conda-forge.

[samcunliffe]

@paddyroddy I think I softly disagree since:

Best: At least one person in ARC uses this. We actively recommend using it above other tools. It is the single recommended tool for a given purpose.

(I don't use it and I don't know anyone in ARC who does ... @dstansby).

If we have a person who uses it then it might be more relevant for researchers to make pixi the default/green. uv can be a close amber with an explaination (like we did for the doc recomendation of MkDocs vs Sphinx). For sure uv >> conda though, so I think this should go in even if only until we get more experience with pixi.

[paddyroddy]

My thought is if we are going to recommend the one tool, then with pixi you have the benefit of being able to install conda-forge isolated dependencies. The problem is neither I nor @samcunliffe have properly used it, so it doesn't feel like it makes the most sense to outright recommend.

[matt-graham]

I also don't have any real experience with Pixi so don't feel qualified to make a judgement as to whether we should recommend or not, but it sounds like we have general agreement (or at least amongst the three of us 😁) that uv ≫ Conda so I'll merge this in now and we can revisit whether to recommend Pixi in future.