Rollup merge of #75106 - etherealist:docs_manifest, r=Mark-Simulacrum

Enable docs on in the x86_64-unknown-linux-musl manifest

Add the rust-docs component to toolchain x86_64-unknown-linux-musl, which allows people using rustup on their musl-based linux distribution to download the rust-docs.

Generating and uploading the docs was enabled in b5d143b (#74871).

In #75102 @Mark-Simulacrum found that we are uploading the docs, but the correct manifest is missing.

* The relevant call to build-manifest seems to be [in bootstrap](c058a8b8dc/src/bootstrap/dist.rs (L2334))

* The manifest is then used in [promote-release crontab](https://github.com/rust-lang/rust-central-station/blob/master/crontab)

src/tools/build-manifest/src/main.rs

@@ -156,6 +156,7 @@ static DOCS_TARGETS: &[&str] = &[
     "x86_64-pc-windows-gnu",
     "x86_64-pc-windows-msvc",
     "x86_64-unknown-linux-gnu",
+    "x86_64-unknown-linux-musl",
 ];
 
 static MINGW: &[&str] = &["i686-pc-windows-gnu", "x86_64-pc-windows-gnu"];