Initial PoC of embassy book

docs/antora.yml

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+name: embassy
+title: Embassy
+version: dev
+nav:
+  - modules/ROOT/nav.adoc

docs/modules/ROOT/nav.adoc

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+* xref:runtime.adoc[Runtime]
+* xref:traits.adoc[Traits]
+* xref:nrf.adoc[Nordic nRF HAL]
+* xref:stm32.adoc[STM32 HAL]

docs/modules/ROOT/pages/index.adoc

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+= Embassy
+
+Embassy is a project to make async/await a first-class option for embedded development.
+
+== Traits and types
+
+`embassy` provides a set of traits and types specifically designed for `async` usage.
+
+* `embassy::io`: `AsyncBufRead`, `AsyncWrite`. Traits for byte-stream IO, essentially `no_std` compatible versions of `futures::io`.
+* `embassy::traits::flash`: Flash device trait.
+* `embassy::time`: `Clock` and `Alarm` traits. Std-like `Duration` and `Instant`.
+* More traits for SPI, I2C, UART async HAL coming soon.
+
+== Executor
+
+The `embassy::executor` module provides an async/await executor designed for embedded usage.
+
+* No `alloc`, no heap needed. Task futures are statically allocated.
+* No "fixed capacity" data structures, executor works with 1 or 1000 tasks without needing config/tuning.
+* Integrated timer queue: sleeping is easy, just do `Timer::after(Duration::from_secs(1)).await;`.
+* No busy-loop polling: CPU sleeps when there's no work to do, using interrupts or `WFE/SEV`.
+* Efficient polling: a wake will only poll the woken task, not all of them.
+* Fair: a task can't monopolize CPU time even if it's constantly being woken. All other tasks get a chance to run before a given task gets polled for the second time.
+* Creating multiple executor instances is supported, to run tasks with multiple priority levels. This allows higher-priority tasks to preempt lower-priority tasks.

docs/modules/ROOT/pages/nrf.adoc

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+= Embassy nRF HAL
+
+TODO

docs/modules/ROOT/pages/runtime.adoc

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+= Embassy runtime
+
+TODO

docs/modules/ROOT/pages/stm32.adoc

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+= Embassy STM32 HAL
+
+TODO

docs/modules/ROOT/pages/traits.adoc

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+= Embassy Traits
+
+TODO