Update docs for LBT

We should add in the documentation that HSL.jl supports LBT and also how to switch a BLAS vendor with LBT.
```julia
using ReferenceBLAS32_jll, LAPACK32_jll
LinearAlgebra.BLAS.lbt_forward(libblas32)
LinearAlgebra.BLAS.lbt_forward(liblapack32)
```

```julia
using blis32_jll, LAPACK32_jll
LinearAlgebra.BLAS.lbt_forward(blis32)
LinearAlgebra.BLAS.lbt_forward(liblapack32)
```

```julia
import LinearAlgebra, OpenBLAS32_jll
LinearAlgebra.BLAS.lbt_forward(OpenBLAS32_jll.libopenblas_path)
```

```julia
import LinearAlgebra, MKL_jll
LinearAlgebra.BLAS.lbt_forward(MKL_jll.libmkl_rt_path)
```

```julia
using AOCL
```

```julia
using AppleAccelerate
```

```julia
import LinearAlgebra, blis_jll, LAPACK_jll
LinearAlgebra.BLAS.lbt_forward(blis)
LinearAlgebra.BLAS.lbt_forward(liblapack)
```

```julia
# To easily reset some BLAS/LAPACK backends.
LinearAlgebra.__init__()
```

```julia
julia> BLAS.get_config()
LinearAlgebra.BLAS.LBTConfig
Libraries:
└ [ILP64] libopenblas64_.s

MKL.__init__()

julia> BLAS.get_config()
LinearAlgebra.BLAS.LBTConfig
Libraries:
├ [ILP64] libmkl_rt.so
└ [ LP64] libmkl_rt.so
```