fix(docs): cache deployment reference build

[jeremi]

Summary

• cache the Rust target used by the docs authoring-reference generator
• use the supported 600-second maximum for actions/deploy-pages
• keep workflow structure coverage for both settings

Why

The latest deployment trace showed that compiling registryctl consumed five minutes of the 5:45 /dev/ build. The prior 1,200-second Pages setting is capped by actions/deploy-pages at 600 seconds, so it emitted a warning and still timed out after ten minutes.

Verification

• python3 -m unittest release/scripts/test_release_workflow_structure.py
• actionlint .github/workflows/docs-pages.yml
• git diff --check

[chatgpt-codex-connector]

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[jeremi]

Resolved the automated DCO false positive: commit 2878a6506c5c6babaec3ecbe71f6522d6ab285d3 includes Signed-off-by: Jeremi Joslin <jeremi@joslin.fr> in both the local commit and GitHub commit API, and the dedicated Check commit sign-offs job passed.