docs: drop completed how-to TODO item

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dfa1

TODO.md

@@ -54,7 +54,6 @@
 ## Documentation
 
 - [ ] Format specification: byte-exact diagrams for file layout and each encoding, with annotated examples (Arrow spec style)
-- [ ] how to use the library and the cli
 
 ## Tooling