Rollup merge of #139885 - jyn514:doc-rustc-bootstrap, r=Nadrieril

document RUSTC_BOOTSTRAP, RUSTC_OVERRIDE_VERSION_STRING, and -Z allow-features in the unstable book

[rendered](https://github.com/jyn514/rust/blob/doc-rustc-bootstrap/src/doc/unstable-book/src/compiler-flags/rustc-bootstrap.md)
mcp: rust-lang/compiler-team#863

Author: matthiaskrgr