Rollup merge of #48972 - mark-i-m:rustdoc_readme, r=QuietMisdreavus
Rollup merge of #48972 - mark-i-m:rustdoc_readme, r=QuietMisdreavus

Move librustdoc readme to rustc guide

cc https://github.com/rust-lang-nursery/rustc-guide/issues/2 and #48478

Don't merge this before https://github.com/rust-lang-nursery/rustc-guide/pull/86

aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust