convert to markmap format

cf. https://markmap.js.org/
- VSCode https://marketplace.visualstudio.com/items?itemName=gera2ld.markmap-vscode
- Mkdocs https://github.com/markmap/mkdocs-markmap
- https://github.com/verocloud/obsidian-mindmap-nextgen

Author: noraj