Move the TODO board to its own page

[zixiaowang17]

Summary

• keep the About homepage focused on Community Activity
• move the formalization TODO board to a dedicated todos.html page
• add TODOs to the main navigation
• keep both pages on the stable snapshot data pipeline introduced in Add a stable, separate TODO board #10

Verification

• homepage snapshot validation
• homepage data extraction tests
• static HTML rebuild
• JavaScript syntax check
• whitespace check