ci: add build-docs workflow

[jimisola]

Summary

• Add caller workflow for centralized Antora docs build validation
• Runs on PRs and pushes to main when docs/** files change
• Calls reusable workflow from reqstool/.github

Test plan

• Verify Build Docs workflow passes after merge

In general, the problem is fixed by explicitly declaring a permissions block that restricts the GITHUB_TOKEN to the least privileges needed. Since this workflow only triggers a reusable workflow and there is no indication it needs write access, a safe minimal default is contents: read at the workflow root. Root‑level permissions apply to all jobs that do not override them, including the build job here.

The single best way to fix this without changing existing functionality is to add a root‑level permissions section immediately after the name: line (line 1). For example:

name: Build Docs
permissions:
  contents: read

This keeps the existing triggers and the build job unchanged and only narrows the default GITHUB_TOKEN privileges for this workflow. No additional imports or methods are needed, since this is a GitHub Actions YAML configuration change only.

Concretely:

• Edit .github/workflows/build-docs.yml.
• Insert a permissions block after line 1 (name: Build Docs), before the on: section.
• Set contents: read as a minimal, least‑privilege default suitable for a documentation build workflow that should not need to push changes or modify repo state.