docs/design/map.md says what /map shows that boards and the dependency list do not, and what to cut #2768

Part of the former #2695 (doc only, no code).

Done:

• [ ] one short doc: each section of /map (src/web/pages/map*), whether boards or an issue's dependency list already shows it, and keep or cut for each
• [ ] a test that every section the doc names still exists in the page source

Files: docs/design/map.md, one test.