Clarify purpose of bevy_dev_tools in crate-level docs

Great question! My stance on "what is bevy_dev_tools":

1. I'd like a unified set of tools that can be used to ease the development of Bevy games, primarily by displaying information about the state of the app.
2. These tools should be easy to discover.
3. These tools should have a unified style and use shared idioms and code.
4. These tools should be able to be toggled on and off by end users in a number of convenient ways using a consistent pattern (e.g. editor command palette, dev console, and so on).

The fact that users *could* use these tools in a final product doesn't negate the fact that they're primarily going to be used for development: there's nothing stopping you from enabling dev tools in your final build but only turning on the key features you need :)

_Originally posted by @alice-i-cecile in https://github.com/bevyengine/bevy/issues/12354#issuecomment-1982307594_