Resolve "Subsite: Software Engineering Consulting Manual"

Closes #350 (closed)

• consulting.html to consulting.md (not feasible)
• Create a specific folder for consulting /consulting-materials/handbook/
• Add link in an existing/non-existing page
• Add /consulting-materials/index.md with some content (if not done on the initial page before) (no extra page required)
• Mark each folder with at least a file .gitkeep
• Adjust fetch_external_content.sh to include the external build *.html site
• Add /consulting-handbook/ to include .gitignore