doc: elaborate on async & threads interop

[kaspar030]

There's a plethora of synchronization primitives in embassy, and some for our threading.
Threads can block_on() embassy (async) futures.

But, how can those be used for reasonable interactions? This needs documentation.

Ideas:

• async impl on executor provides blocking impl for threads
  • provide blocking embedded-hal impls for threads that make use of the async variant within the executors
  • similar for embedded-nal
  • this would make threads really useful, as non-async code could seamlessly coexist with async code
• do we need another channel that works for both threads and tasks?
• which mutexes work, how do they interact? interrupt executor vs. thread executor? PIP?
• ...

[ROMemories]

Beginning of answer regarding mutexes here: #398 (comment)