docs(geometry): link strain helpers and fix LICENSE URL

Update the SPDX banner to `LICENSE.md` and cross-reference `extract_strain`
from the small-strain docstring so readers know where gradients are consumed.

src/geometry/strain.jl

@@ -1,5 +1,5 @@
 # This file is part of JuliaFEM.
-# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
 
 """
 Strain computation utilities for finite element analysis.
@@ -24,6 +24,9 @@ Small strain: ε = ½(∇u + ∇u^T) where ∇u = ∑ᵢ uᵢ ⊗ (dNᵢ/dx)
 # Returns
 - `ε::SymmetricTensor{2,3}`: Small strain tensor
 
+See also [`extract_strain`](@ref), which starts from an already assembled
+displacement-gradient tensor `∇u`.
+
 # Example
 ```julia
 u = (Vec{3}((0.1, 0.0, 0.0)), Vec{3}((0.15, 0.02, 0.0)))