Rollup merge of #70782 - faern:use-assoc-float-consts, r=dtolnay

Stop importing the float modules in documentation

Follow up to #69860. I realized I had not searched for and fixed this for the float values. So with this PR they also use the associated constants instead of the module level constants.

For the documentation where it also was using the `consts` submodule I opted to change it to import that directly. This becomes more in line with how other docs that use the `consts` submodule looks. And it also makes it so there are not two `f32` or `f64` things in the current namespace (both the module and the primitive type) and then hopefully confusing documentation readers less.

r? @dtolnay