Migrate doc(cfg) to new Attribute API

Author: GuillaumeGomez

Cargo.lock

@@ -4869,6 +4869,7 @@ dependencies = [
  "minifier",
  "pulldown-cmark-escape",
  "regex",
+ "rustc_proc_macro",
  "rustdoc-json-types",
  "serde",
  "serde_json",

src/librustdoc/Cargo.toml

@@ -18,6 +18,7 @@ minifier = { version = "0.3.5", default-features = false }
 pulldown-cmark-escape = { version = "0.11.0", features = ["simd"] }
 regex = "1"
 rustdoc-json-types = { path = "../rustdoc-json-types" }
+rustc_proc_macro = { path = "../../compiler/rustc_proc_macro" }
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 smallvec = "1.8.1"