Auto merge of #45295 - Technius:docs/process, r=steveklabnik
Auto merge of #45295 - Technius:docs/process, r=steveklabnik

Improve std::process module docs

Addresses part of #29370

I've changed the first `cat` example to a "Hello World" example involving echo, and I've also added another example showing how to pipe output. I'm still working on the module-level description.

For now, I'd like feedback on the examples.

r? @steveklabnik

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