Auto merge of #134907 - Kobzol:rustc-dev-guide-josh, r=ehuss

Turn rustc-dev-guide into a Josh subtree

Discussed on [Zulip](https://rust-lang.zulipchat.com/#narrow/channel/196385-t-compiler.2Fwg-rustc-dev-guide/topic/a.20move.20to.20main.20repo.20.28rust-lang.2Frust.29).

Accompanying rustc-dev-guide PR: rust-lang/rustc-dev-guide#2183

I didn't create a bootstrap step for rustc-dev-guide yet, because the rustc-dev-guide version that we currently use in this repo doesn't have linkcheck enabled and that fails tests.

The subtree starts with commit [ad93c5f1c49f2aeb45f7a4954017b1e607df9f5e](rust-lang/rustc-dev-guide@ad93c5f).

What I did:
```
export DIR=src/doc/rustc-dev-guide

# Remove submodule
git submodule status ${DIR}
git submodule deinit ${DIR}
git rm -r --cached ${DIR}
rm -rf ${DIR}
# Remove rustc-dev-guide from .gitmodules
git commit -m"Removed `${DIR}` submodule"

# Import history with josh
git fetch https://github.com/rust-lang/rustc-dev-guide ad93c5f1c49f2aeb45f7a4954017b1e607df9f5e
josh-filter ':prefix=src/doc/rustc-dev-guide' FETCH_HEAD
git merge --allow-unrelated FILTERED_HEAD

# A few follow-up cleanup commits
```

r? ehuss

Author: bors

.gitmodules

@@ -22,10 +22,6 @@
 	path = library/stdarch
 	url = https://github.com/rust-lang/stdarch.git
 	shallow = true
-[submodule "src/doc/rustc-dev-guide"]
-	path = src/doc/rustc-dev-guide
-	url = https://github.com/rust-lang/rustc-dev-guide.git
-	shallow = true
 [submodule "src/doc/edition-guide"]
 	path = src/doc/edition-guide
 	url = https://github.com/rust-lang/edition-guide.git

src/bootstrap/src/core/build_steps/doc.rs

@@ -63,7 +63,7 @@ macro_rules! book {
 }
 
 // NOTE: When adding a book here, make sure to ALSO build the book by
-// adding a build step in `src/bootstrap/builder.rs`!
+// adding a build step in `src/bootstrap/code/builder/mod.rs`!
 // NOTE: Make sure to add the corresponding submodule when adding a new book.
 // FIXME: Make checking for a submodule automatic somehow (maybe by having a list of all submodules
 // and checking against it?).