Auto merge of #37763 - liigo:rustdoc-playground, r=alexcrichton

rustdoc: add cli argument `--playground-url`

Add a new cli argument `--playground-url` for rustdoc:

`rustdoc lib.rs --playground-url="https://play.rust-lang.org/"`

`rustdoc book.md --playground-url="https://play.rust-lang.org/"`

This makes it possible for tools like https://docs.rs to generate crate docs that can submit samples code to run at https://play.rust-lang.org, even if the crate's author *forgot* putting `#![doc(html_playground_url = "https://play.rust-lang.org/")]` to crate root. By the way, I'd like to say, many crate authors are not aware of existing of `#![doc(html_playground_url = "https://play.rust-lang.org/")]`.

`--playground-url` may be reset by `--markdown-playground-url` or `#![doc(html_playground_url=...)]`, so it's backward compatible.

@alexcrichton since you implemented playground-url related features.

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