ci: add GitHub Actions CI and release workflows

Add ci.yml running fmt, clippy, build, and test on stable + nightly
Rust for PRs and pushes to main. Add release.yml for automated crate
publishing to crates.io on version tag pushes. Remove dead_code allow
and add crate-level doc comment with usage example.

src/lib.rs

@@ -1,5 +1,21 @@
-#![allow(dead_code)]
-
+/// Rust SDK for the Onspring API v2.
+///
+/// # Example
+///
+/// ```no_run
+/// use onspring::{OnspringClient, PagingRequest};
+///
+/// # async fn example() -> onspring::Result<()> {
+/// let client = OnspringClient::builder("your-api-key").build();
+///
+/// // Check connectivity
+/// client.ping().await?;
+///
+/// // List apps
+/// let apps = client.list_apps(None).await?;
+/// # Ok(())
+/// # }
+/// ```
 pub mod client;
 mod endpoints;
 pub mod error;