Change engine tagline from "a refreshingly simple data-driven engine written in Rust"

I mentioned this in [this review comment](https://github.com/bevyengine/bevy-website/pull/2233/files#r2370801031), which @cart said he was open to doing.

Our current tagline is "Bevy is a refreshingly simple data-driven game engine". Let's break that down!

## Refreshingly simple

My major complaint is that I'm not sure that "refreshingly simple" is true these days, especially for devs coming from other languages and paradigms.

Instead, I would prefer if we could focus on one or two of:

- modular or flexible
- code-first
- fast
- refactorable? I think this is really valuable, but doesn't exactly roll-off the tongue
- ECS-first??

## Data-driven

No one knows what this means. In theory, this means "you can change behavior by modifying assets on disk". Which is great! And a goal we have! But it's not a reality today.

The other problem is that this is easily confused with "data-oriented", which means "designed in a way that is performant because of smart data layout". This is what an ECS does, but it's also jargon.

## Game engine

I mean, technically :p A lot of our users aren't making games! It's still a guiding star though.

People also get mad because "you can't have a game engine without an editor", but I think they're just wrong.

## Written in Rust

People gripe about this ("it's about the product, not the technology used to make it!!"), but I think it's important to communicate this upfront, because Rust is what they're going to be writing.