Update no_std docs

This is part of #28572, but doesn't complete it. Amongst other things,
this patch:

* Increases consistency in the way feature flags are used with other
  docs.
* Removes the ignores, which is nice: we actually had some syntax errors
  in the examples :sob:.
* Mentions #![no_core]

Realistically, this document used to be in the order of least to most:
nothing, then adding core. But with the changes in RFC 1184, this is
backwards: it now shows stuff that uses core from the beginning. In the
future, I'd like to revamp this to go from 'most to least', but I'd like
to see the discussion in https://github.com/rust-lang/rust/issues/27701
goes before I write more.

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