Rollup merge of #105465 - jyn514:docs, r=Mark-Simulacrum

Rollup merge of #105465 - jyn514:docs, r=Mark-Simulacrum

Improve top-level docs

See a detailed explanation in the commit messages. This is a companion PR to https://github.com/rust-lang/rustc-dev-guide/pull/1528.

* Link to other resources instead of inlining their information
* Remove ancient and outdated reference to `config.mk`
* Suggest `profile = "user"` in the README
* Add detail about dependencies from the dev-guide
* Link to CONTRIBUTING.md instead of rustc-dev-guide in the main readme
* Link to `std-dev-guide` in CONTRIBUTING.md