docs: backfill missing CLI documentation

[marcus]

Summary

• align the Docusaurus command reference with the current Cobra help surface
• add focused user-facing docs for notes, sync/collaboration, and system diagnostics
• wire new pages into the sidebar and fix stale README documentation references

Validation

• npm run build (website)
• git diff --cached --check before commit
• go vet and go build passed via pre-commit hook

Notes

• Sync role examples use only the accepted roles: owner, writer, reader.
• The diagnostics docs avoid unsupported td doctor flags; doctor is documented as help-only for flags beyond the command itself.
• Go tests were not run separately because this is a docs-only change verified against Cobra help output.