Auto merge of #145011 - Kobzol:bootstrap-doc, r=jieyouxu

Enforce in bootstrap that doc must have stage at least 1

Following with the bootstrap cleanups, this time around `doc` steps. Should be pretty straightforward, because the supporting infrastructure was already there.

The only thing I found a bit fishy is using `Mode::ToolBootstrap` as a "catch-all" mode for non-rustc-private steps in `tool_doc!`, but I don't think that we need to distinguish the tools in some special way when documenting them, apart from supporting `rustc_private`.

Before, `x doc` more or less defaulted to what we call stage 2 now. Now it is properly stage 1, so e.g. `x doc compiler` documents the compiler using the stage0/beta rust(do)c.

r? `@jieyouxu`

try-job: dist-aarch64-msvc

Author: bors