docs CI: fix gh-pages deploy (GITHUB_TOKEN write permissions, drop broken DOCUMENTER_KEY)

[ChrisRackauckas-Claude]

Problem

The Documentation workflow's docs build passes, but the deploydocs gh-pages push fails. Two compounding causes:

1. The build-and-deploy job had no permissions: block, so the GITHUB_TOKEN flowing into the job is read-only and cannot push to gh-pages.
2. The build step injected DOCUMENTER_KEY: ${{ secrets.DOCUMENTER_KEY }}. When that env var is present, Documenter's deploydocs takes the SSH deploy path (git@github.com:...), which fails with Permission denied (publickey) because the deploy key is missing/invalid.

Fix (YAML-only)

• Add a permissions: block to the build-and-deploy job:

  permissions:
    actions: write
    contents: write
    statuses: write

  This mirrors the proven pattern from SciML/OrdinaryDiffEqOperatorSplitting.jl Update optimize! method for LeastSquaresOptim case #90.
• Remove DOCUMENTER_KEY from the build step's env. With no DOCUMENTER_KEY present, deploydocs auto-falls back to pushing over HTTPS using GITHUB_TOKEN, which the new contents: write permission now authorizes. GITHUB_TOKEN is retained in the env. No secret is required.

deploydocs(repo = "github.com/SciML/DiffEqDocs.jl.git") in docs/make.jl is already the HTTPS form, so the token path works as-is.

Verification

YAML validated with python3 (yaml.safe_load) — parses cleanly; permissions resolves to {actions: write, contents: write, statuses: write}. The full deploy cannot be verified without a CI run (it requires the self-hosted runner + a push to master), but the change matches the proven #90 pattern and the documented Documenter token-deploy fallback.

This does not disable the docs check or set warnonly.

Please ignore until reviewed by @ChrisRackauckas