docs(ci): add phase-1 build-docs command checks

[WuXieSec]

Summary

Implements a phase-1 automated test for build documentation commands.

What this PR adds

• CI workflow: .github/workflows/docs-build-command-check.yml
• Checker script: tools/check_build_docs_commands.py

Behavior

• Scans docs/variants/**/building-manual.md
• Extracts shell/bash code blocks
• Runs non-executing syntax validation (bash -n) to catch broken snippets early
• Skips placeholder/pseudocode snippets (<version>, <variant>, (docker), etc.)
• Emits warnings for uncommon command prefixes (informational, non-blocking)

Why this approach

For #1153 this provides a low-risk first step that is CI-safe and immediately useful, while keeping room for future phases that execute selected commands in controlled environments.

Related issue: Dasharo/dasharo-issues#1153