Cleanup left-overs from new UI migration

[benoit74]

Before releasing 4.0, we should cleanup left-overs from the migration:

• most code in scraper/src/gutenberg2zim/templates is now unused
• scraper/get_js_deps.sh
• GUTENBERG_STRUCTURE.md and scraper/docs/JSON_FILE_STRUCTURE.md files were more a reference for the migration ; I however feel like most info is very relevant and could definitely help a newcomer ; I'm just a bit worried about its maintenance ; either we clean it a bit (hoping it will be maintained ... not sure at all about that) or we simply remove it (which is a bit sad to loose such info ... but could easily be rebuild with AI)
• maybe other bits I forgot