ignore build folder and update documentation

AGENTS.md

@@ -23,6 +23,7 @@ This project builds a presentation deck for Marp based on Markdown files.
 - Use `slides/` directory for Markdown slide files
 - Use `assets/` for images and media files
 - Use `themes/` for custom Marp themes
+- Use `build/` directory for generated output files (ignored by git)
 
 ### Naming Conventions
 - Slide files: `index.md`, `additional-slides.md` (descriptive names)