doc(port_std): add a crate-level documentation

Author: yvt

src/constance_port_std/src/lib.md

@@ -0,0 +1,32 @@
+Simulator for running [`::constance`] on a hosted environment
+
+# Usage
+
+```rust
+#![feature(const_loop)]
+#![feature(const_fn)]
+#![feature(const_if_match)]
+#![feature(const_mut_refs)]
+
+// Require `unsafe` even in `unsafe fn` - highly recommended
+#![feature(unsafe_block_in_unsafe_fn)]
+#![deny(unsafe_op_in_unsafe_fn)]
+
+use constance::kernel::Task;
+
+// Use the simulator port
+constance_port_std::use_port!(unsafe struct System);
+
+const COTTAGE: () = constance::build!(System, configure_app);
+
+constance::configure! {
+    const fn configure_app(_: &mut CfgBuilder<System>) -> () {
+        new! { Task<_>, start = task_body, priority = 1, active = true };
+    }
+}
+
+fn task_body(_: usize) {
+    // The simulator initializes `env_logger` automatically
+    log::warn!("yay");
+}
+```

src/constance_port_std/src/lib.rs

@@ -1,5 +1,7 @@
 #![feature(const_fn)]
+#![feature(external_doc)]
 #![feature(unsafe_block_in_unsafe_fn)] // `unsafe fn` doesn't imply `unsafe {}`
+#![doc(include = "./lib.md")]
 #![deny(unsafe_op_in_unsafe_fn)]
 use atomic_ref::AtomicRef;
 use constance::{prelude::*, utils::intrusive_list::StaticListHead};