This site was converted from https://sites.google.com/site/xiangyangsite before Google forced it to be migrated to their new site template (which doesn't have many features the old one had, such as subpage listing, page attachment, etc.)
Install mercury-parser with
npm -g install @postlight/mercury-parser
Download a mark down converter called reader.
Assume in the reader repository, run
mercury-parser https://example.com/path | ./reader.py -f md - > path.md