[feat] add example scripts for transcription, model downloading, and updates; improve documentation with guides for CI, packaging, and development

src/backend.rs

@@ -1,3 +1,4 @@
+//! Transcription backend selection and implementations (CPU/GPU) used by PolyScribe.
 use crate::OutputEntry;
 use crate::{decode_audio_to_pcm_f32_ffmpeg, find_model_file};
 use anyhow::{Context, Result, anyhow};