Minor documentation improvements
- Remove reference to Meio until it's actually working with stable
versions of Eio.
- Remove the "Please try porting your programs" request now that Eio is
stable.
- Use a simpler and more typical example for the switches section.
- In the `Executor_pool` make the second example create the pool the
same way as the first one (and shrink it a bit).
- Replace out-of-date retro-httpaf-bench example with cohttp-eio and
capnp-rpc examples.
- In `Net.run_server`, suggest using `Executor_pool`.
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