docs: make the permutation_init doctests deterministic (drop the duplicate docs workflow)

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas.

What changed and why

permutation_init returns a randomly permuted matrix, but two jldoctest blocks in the permutation_init docstring printed that matrix, so the expected output depended on the draw. They now assert the structure instead:

julia> m = permutation_init(5, 5);

julia> size(m) == (5, 5) && count(!iszero, m) == 5 && sort(vec(m[m .!= 0])) == fill(0.1f0, 5)
true

Correction: an earlier revision of this PR also added a .github/workflows/Documentation.yml built on ubuntu-latest. That was wrong and has been removed. ReservoirComputing already builds its documentation on a self-hosted GPU runner — the gpu-docs job in .github/workflows/GPU.yml, whose workflow is literally named "GPU Documentation", runs-on: [self-hosted, Linux, X64, gpu] — and that job is green on master, so there was no docs build to repair.

Verification

• master's GPU docs job is green: run https://github.com/SciML/ReservoirComputing.jl/actions/runs/35146312301 → Documentation=success.
• The two rewritten doctests are deterministic: the assertion checks shape, the number of nonzeros and the sorted nonzero values, none of which depend on the permutation.
• grep -rl 'docs/make.jl' .github/workflows matches only GPU.yml after this change.

Not verified

• The GPU docs job on this branch (draft PRs skip it); the doctest content change itself is covered by the existing green GPU build.
• Whether the RNG dependence was causing visible flakiness in CI; it is a latent flake that this removes.

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