[codex] Pin docs setup-julia action to v3

[bernalde]

Pins the Documentation workflow's Julia setup action to the stable v3 major tag.

Why:

• The post-merge Documentation run for PR [codex] Update checkout action for Node 24 #91 still emitted the GitHub Actions Node 20 deprecation warning because docs used julia-actions/setup-julia@latest.
• CI and Benchmark already use julia-actions/setup-julia@v3 successfully.

Validation:

• git diff --check
• scanned workflows for remaining actions/checkout@v4 and julia-actions/setup-julia@latest references

[github-actions]

Performance Report - main vs. dev

Timings are reported for the batched benchmark operations as minimum (median) ± std, and comparison uses a 15.00% tolerance to reduce false positives on shared runners.

case	main	dev	diff
/constructors/n=128/Model/MOI/bool	847.390 μs (925.124 μs) ± 668.331 μs	832.763 μs (906.811 μs) ± 598.492 μs	invariant (-1.73%)
/constructors/n=128/Model	658.873 μs (703.682 μs) ± 993.571 μs	652.275 μs (697.499 μs) ± 922.602 μs	invariant (-1.00%)
/constructors/n=2048/Model/MOI/bool	36.155 ms (38.186 ms) ± 41.257 ms	35.747 ms (37.547 ms) ± 40.923 ms	invariant (-1.13%)
/constructors/n=2048/Model	28.293 ms (32.395 ms) ± 23.141 ms	28.045 ms (32.064 ms) ± 21.498 ms	invariant (-0.88%)
/constructors/n=384/Model/MOI/bool	3.184 ms (3.348 ms) ± 741.234 μs	3.090 ms (3.303 ms) ± 668.067 μs	invariant (-2.97%)
/constructors/n=384/Model	2.680 ms (2.833 ms) ± 1.630 ms	2.668 ms (2.836 ms) ± 1.645 ms	invariant (-0.47%)
/conversions/n=128/form/Dict	285.126 μs (373.146 μs) ± 2.889 ms	269.630 μs (355.153 μs) ± 2.774 ms	invariant (-5.43%)
/conversions/n=128/form/Matrix/Float32	51.842 μs (73.779 μs) ± 221.314 μs	45.903 μs (70.326 μs) ± 205.200 μs	invariant (-11.46%)
/conversions/n=128/form/Matrix	71.793 μs (111.597 μs) ± 2.817 ms	59.837 μs (86.546 μs) ± 2.692 ms	improvement (-16.65%)
/conversions/n=128/form/SparseMatrixCSC	168.836 μs (217.595 μs) ± 299.863 μs	159.410 μs (183.179 μs) ± 265.889 μs	invariant (-5.58%)
/conversions/n=128/ising/Matrix	67.381 μs (101.606 μs) ± 2.841 ms	64.373 μs (104.435 μs) ± 2.708 ms	invariant (-4.46%)
/conversions/n=128/qubo/Matrix/min	709.804 μs (834.865 μs) ± 5.960 ms	682.968 μs (825.138 μs) ± 7.197 ms	invariant (-3.78%)
/conversions/n=128/qubo/Matrix	393.155 μs (486.838 μs) ± 4.648 ms	370.414 μs (448.988 μs) ± 5.386 ms	invariant (-5.78%)
/conversions/n=384/form/Dict	1.054 ms (1.161 ms) ± 4.610 ms	784.899 μs (1.080 ms) ± 4.163 ms	improvement (-25.51%)
/conversions/n=384/form/Matrix/Float32	262.642 μs (489.221 μs) ± 11.497 ms	236.442 μs (477.254 μs) ± 10.767 ms	invariant (-9.98%)
/conversions/n=384/form/Matrix	450.034 μs (911.924 μs) ± 18.408 ms	411.066 μs (895.538 μs) ± 17.458 ms	invariant (-8.66%)
/conversions/n=384/form/SparseMatrixCSC	1.427 ms (1.947 ms) ± 9.165 ms	1.343 ms (1.900 ms) ± 8.431 ms	invariant (-5.94%)
/conversions/n=384/ising/Matrix	436.195 μs (906.884 μs) ± 18.095 ms	411.803 μs (884.271 μs) ± 17.064 ms	invariant (-5.59%)
/conversions/n=384/qubo/Matrix/min	8.051 ms (8.637 ms) ± 36.350 ms	7.425 ms (8.374 ms) ± 38.757 ms	invariant (-7.77%)
/conversions/n=384/qubo/Matrix	3.854 ms (4.708 ms) ± 28.708 ms	3.497 ms (4.620 ms) ± 27.615 ms	invariant (-9.26%)
/evaluation/n=128/value/dense-form	446.188 μs (469.200 μs) ± 519.018 μs	446.056 μs (467.175 μs) ± 445.687 μs	invariant (-0.03%)
/evaluation/n=128/value/dict-form	260.001 μs (260.663 μs) ± 4.327 μs	266.133 μs (267.819 μs) ± 10.778 μs	invariant (2.36%)
/evaluation/n=128/value/model	52.703 μs (74.730 μs) ± 455.054 μs	52.747 μs (54.794 μs) ± 333.072 μs	invariant (0.08%)
/evaluation/n=128/value/sparse-form	52.286 μs (74.430 μs) ± 424.361 μs	52.509 μs (61.324 μs) ± 401.836 μs	invariant (0.43%)
/evaluation/n=384/value/dense-form	4.929 ms (4.942 ms) ± 277.949 μs	4.935 ms (4.948 ms) ± 233.668 μs	invariant (0.12%)
/evaluation/n=384/value/dict-form	479.500 μs (482.390 μs) ± 8.727 μs	501.132 μs (505.863 μs) ± 6.622 μs	invariant (4.51%)
/evaluation/n=384/value/model	185.180 μs (191.563 μs) ± 231.225 μs	180.639 μs (193.260 μs) ± 201.306 μs	invariant (-2.45%)
/evaluation/n=384/value/sparse-form	186.937 μs (193.108 μs) ± 253.028 μs	186.153 μs (193.843 μs) ± 226.070 μs	invariant (-0.42%)