docs: document remaining config keys

[CooperSheroy]

Summary

• add single_file_view and username to the full config example
• document their defaults and behavior in the config options table

Verification

• git diff --check
• direct inspection against src/config/mod.rs accepted keys

Note: cargo test config::tests::should_parse_single_file_view_true config::tests::should_parse_username --lib could not run locally because this cron environment has no installed Rust stable toolchain (cargo missing; rustup run stable reports stable-aarch64-apple-darwin is not installed).