Documentation on GPU support

I tried to use `Interpolations.jl` with CUDA and found myself deeply lost in the documentation and didn't know what to expect from the package. I found about GPU support from GitHub issues (and PR #504). But that's pretty much all the information available. All the documentation I could about GPU support is a [short section](https://juliamath.github.io/Interpolations.jl/stable/devdocs/#GPU-Support) in the "Developer documentation".

As I user, I would like to use interpolants from interpolations.jl in my CUDA kernels. The naive attempt of not doing anything special leads to functions that do not compile
```
.T is of type Interpolations.ScaledInterpolation{Float64, 1, Interpolations.BSplineInterpolation{Float64, 1, Vector{Float64}, Interpolations.BSpline{Interpolations.Linear{Interpolations.Throw{Interpolations.OnGrid}}}, Tuple{Base.OneTo{Int64}}}, Interpolations.BSpline{Interpolations.Linear{Interpolations.Throw{Interpolations.OnGrid}}}, Tuple{StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}, Int64}}} which is not isbits.
.itp is of type Interpolations.BSplineInterpolation{Float64, 1, Vector{Float64}, Interpolations.BSpline{Interpolations.Linear{Interpolations.Throw{Interpolations.OnGrid}}}, Tuple{Base.OneTo{Int64}}} which is not isbits.
.coefs is of type Vector{Float64} which is not isbits.
.u is of type Interpolations.ScaledInterpolation{Float64, 1, Interpolations.BSplineInterpolation{Float64, 1, Vector{Float64}, Interpolations.BSpline{Interpolations.Linear{Interpolations.Throw{Interpolations.OnGrid}}}, Tuple{Base.OneTo{Int64}}}, Interpolations.BSpline{Interpolations.Linear{Interpolations.Throw{Interpolations.OnGrid}}}, Tuple{StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}, Int64}}} which is not isbits.
.itp is of type Interpolations.BSplineInterpolation{Float64, 1, Vector{Float64}, Interpolations.BSpline{Interpolations.Linear{Interpolations.Throw{Interpolations.OnGrid}}}, Tuple{Base.OneTo{Int64}}} which is not isbits.
.coefs is of type Vector{Float64} which is not isbits.
.q is of type Interpolations.ScaledInterpolation{Float64, 1, Interpolations.BSplineInterpolation{Float64, 1, Vector{Float64}, Interpolations.BSpline{Interpolations.Linear{Interpolations.Throw{Interpolations.OnGrid}}}, Tuple{Base.OneTo{Int64}}}, Interpolations.BSpline{Interpolations.Linear{Interpolations.Throw{Interpolations.OnGrid}}}, Tuple{StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}, Int64}}} which is not isbits.
.itp is of type Interpolations.BSplineInterpolation{Float64, 1, Vector{Float64}, Interpolations.BSpline{Interpolations.Linear{Interpolations.Throw{Interpolations.OnGrid}}}, Tuple{Base.OneTo{Int64}}} which is not isbits.
.coefs is of type Vector{Float64} which is not isbits.
.P is of type Interpolations.ScaledInterpolation{Float64, 1, Interpolations.BSplineInterpolation{Float64, 1, Vector{Float64}, Interpolations.BSpline{Interpolations.Linear{Interpolations.Throw{Interpolations.OnGrid}}}, Tuple{Base.OneTo{Int64}}}, Interpolations.BSpline{Interpolations.Linear{Interpolations.Throw{Interpolations.OnGrid}}}, Tuple{StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}, Int64}}} which is not isbits.
.itp is of type Interpolations.BSplineInterpolation{Float64, 1, Vector{Float64}, Interpolations.BSpline{Interpolations.Linear{Interpolations.Throw{Interpolations.OnGrid}}}, Tuple{Base.OneTo{Int64}}} which is not isbits.
.coefs is of type Vector{Float64} which is not isbits.
.c_co2 is of type Interpolations.ScaledInterpolation{Float64, 1, Interpolations.BSplineInterpolation{Float64, 1, Vector{Float64}, Interpolations.BSpline{Interpolations.Linear{Interpolations.Throw{Interpolations.OnGrid}}}, Tuple{Base.OneTo{Int64}}}, Interpolations.BSpline{Interpolations.Linear{Interpolations.Throw{Interpolations.OnGrid}}}, Tuple{StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}, Int64}}} which is not isbits.
.itp is of type Interpolations.BSplineInterpolation{Float64, 1, Vector{Float64}, Interpolations.BSpline{Interpolations.Linear{Interpolations.Throw{Interpolations.OnGrid}}}, Tuple{Base.OneTo{Int64}}} which is not isbits.
.coefs is of type Vector{Float64} which is not isbits.
```
I tried a bunch of things that didn't work, like changing constructors, or passing `CuArray`s to them.

Following the developer documentation, I managed to have a working function using `adapt` (which I found a little surprising, since I was expecting `adapt` to be only needed on the `Interpolations.jl`-side).

Some of the functions (e.g., `adapt(CuArray{Float64}, itp)` error out on printing (or, more specifically, they to scalar indexing on GPUs).

`cuitp` doesn't work on `Vector`s , or on scalars:
```julia
cuitp.(1:0.5:2 |> collect) # .x is of type Vector{Float64} which is not isbits.
cuitp.(1:0.5:2 |> collect |> CuArray) # This is fine
cuitp(1) # Scalar indexing
cuitp.(Ref(1)) # This is fine
```
In this, I also found unclear if the higher-level constructors supported GPUs or not.

It would be very useful to clearly specify what does it mean for `Interpolations.jl` to support GPUs.