Add format indent for generated code link with rust-playground

[chenyukang]

This idea is from #119161

When clicking the Run button, will jump to the rust-playground, the code is not indent well:

the link is generated from code:

rust/src/librustdoc/html/markdown.rs

Line 306 in 0a5b998

doctest::make_test(&test, krate, false, &Default::default(), edition, None);

I would be nice if we add extra indent space for the code block after inserting fn main { for the code in document.

I checked rust-playgrund, seems there is no a URL argument like format=true for the code link.