documentation.yml: set permissions (contents: write) in the reusable (fix docs-deploy at the source)

[ChrisRackauckas-Claude]

Per @ChrisRackauckas: fix the docs-deploy gh-pages 403 centrally instead of per-repo.

The reusable documentation.yml deploys via GITHUB_TOKEN (when no DOCUMENTER_KEY), which needs contents: write. It declared no permissions:, so each consumer's caller job had to grant write — and the CI-centralization migration dropped those per-repo blocks, 403'ing gh-pages across many repos (CatalystNetworkAnalysis, ColPrac, Evolutionary, PDEBase, SparseColumnPivotedQR, OrdinaryDiffEqOperatorSplitting, …).

This adds a workflow-level permissions: {contents: write, actions: write, statuses: write} to the reusable, so all consumers get it without per-repo blocks. Mirrors the grant that the per-caller fix (OrdinaryDiffEqOperatorSplitting #90, verified → Documentation green) used — just moved to the source.

Needs a v1 retag to take effect. Once retagged, the ~6 per-repo permissions-block PRs I opened can be closed as superseded.

⚠️ Caveat: GitHub gates a reusable's token by the caller — this works as long as the consumer caller doesn't restrict permissions (SciML callers don't) and the org token max allows write (it does — #90's explicit grant succeeded). If any repo's org/caller hard-caps to read-only, that repo would still need its own block or the org Workflow-permissions setting.

Please ignore until reviewed by @ChrisRackauckas.