docs(snippet): Add minimal elasticity quickstart source

Runnable 4×4×4 Hex8 linear elasticity assembly used as the Documenter/README
minimal example source of truth; returns (; ndofs, nnz_stiffness).

- Intended for @literalinclude, test/docs, and scripts/verify_docs_quickstart.jl

docs/src/snippets/minimal_elasticity_quickstart.jl

@@ -0,0 +1,34 @@
+# Source of truth for the package Documenter home page "minimal elasticity" demo.
+# The README.md block under "A modern minimal example" must match this assembly
+# pipeline; `test/docs/runtests.jl` runs both this file and the README fence.
+# Executed by `scripts/verify_docs_quickstart.jl`, `test/docs/runtests.jl`, and
+# (via Documenter) shown on `docs/src/index.md` with `@literalinclude`.
+using JuliaFEM
+using SparseArrays
+
+function minimal_elasticity_quickstart()
+    mesh = create_structured_box_mesh(Hex8;
+        xmin = 0.0, xmax = 1.0, nx = 4,
+        ymin = 0.0, ymax = 1.0, ny = 4,
+        zmin = 0.0, zmax = 1.0, nz = 4,
+    )
+    S  = @DOFSet{u::DOF{Displacement{3}, Vertex}}
+    ET = Element{Hex8, Lagrange{1}, S}
+    elements, handler = create_elements!(mesh, ET)
+
+    material = LinearElastic(E = 210e9, ν = 0.3)
+    kernel   = ContinuumKernel(ContinuumFormulation{FullThreeD}(),
+                               material, Displacement{3}())
+
+    asm   = DOFBasedCOOAssembler()
+    cache = create_cache(asm, elements, handler, mesh, kernel)
+    assemble!(cache, asm, kernel, mesh)
+    K, f  = extract_system(cache)
+
+    nd = size(K, 1)
+    @assert nd > 0
+    @assert size(K, 2) == nd
+    @assert nnz(K) > 0
+    @assert length(f) == nd
+    return (; ndofs = nd, nnz_stiffness = nnz(K))
+end