Preserve mdbook build failures in CI

[0xjc65eth]

Summary

• make mdbook CI steps use pipefail when teeing build output
• build docs/book and docs/reference once each while keeping logs visible in the job output
• remove grep-based ERROR handling that could miss non-ERROR failures or mask command status

Why
The mdbook build job was piping output through tee without preserving the mdbook exit code, and the reference book was built twice. If mdbook failed without a matching ERROR line, the step could still exit successfully or make the failure harder to diagnose.

Testing

• git diff --check
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/ci.yml"); puts "yaml ok"'\n\nCloses ci: handle mdbook build failures better #3887.

[cursor]

PR Summary

Low Risk
Only .github/workflows/ci.yml changes in the docs build job; no application or auth logic.

Overview
Updates the build-mdbook job so doc builds fail reliably and run once per book.

The old steps piped mdbook build through tee without pipefail, then treated success as “no ERROR in the log.” That could let real mdbook failures slip through when output didn’t match that pattern. The reference book was also built twice (once discarded, once captured).

Each book now has a dedicated step (docs/book with strict forc-documenter, docs/reference) using set -o pipefail and tee to /tmp/mdbook-*.log, so the step fails on mdbook’s exit code while logs stay in the job output.

^{Reviewed by Cursor Bugbot for commit a572e46. Bugbot is set up for automated code reviews on this repo. Configure here.}

[fuel-cla-bot]

Thanks for the contribution! Before we can merge this, we need @0xjc65eth to sign the Fuel Labs Contributor License Agreement.