Move rustdoc-related code to rustdoc crate (#12945)

This PR moves the rustdoc-related code out of `html_to_markdown` and
into the `rustdoc` crate.

Release Notes:

- N/A

crates/rustdoc/src/item.rs

@@ -0,0 +1,71 @@
+use std::sync::Arc;
+
+use strum::EnumIter;
+
+#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Copy, EnumIter)]
+pub enum RustdocItemKind {
+    Mod,
+    Macro,
+    Struct,
+    Enum,
+    Constant,
+    Trait,
+    Function,
+    TypeAlias,
+    AttributeMacro,
+    DeriveMacro,
+}
+
+impl RustdocItemKind {
+    pub(crate) const fn class(&self) -> &'static str {
+        match self {
+            Self::Mod => "mod",
+            Self::Macro => "macro",
+            Self::Struct => "struct",
+            Self::Enum => "enum",
+            Self::Constant => "constant",
+            Self::Trait => "trait",
+            Self::Function => "fn",
+            Self::TypeAlias => "type",
+            Self::AttributeMacro => "attr",
+            Self::DeriveMacro => "derive",
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone)]
+pub struct RustdocItem {
+    pub kind: RustdocItemKind,
+    /// The item path, up until the name of the item.
+    pub path: Vec<Arc<str>>,
+    /// The name of the item.
+    pub name: Arc<str>,
+}
+
+impl RustdocItem {
+    pub fn url_path(&self) -> String {
+        let name = &self.name;
+        let mut path_components = self.path.clone();
+
+        match self.kind {
+            RustdocItemKind::Mod => {
+                path_components.push(name.clone());
+                path_components.push("index.html".into());
+            }
+            RustdocItemKind::Macro
+            | RustdocItemKind::Struct
+            | RustdocItemKind::Enum
+            | RustdocItemKind::Constant
+            | RustdocItemKind::Trait
+            | RustdocItemKind::Function
+            | RustdocItemKind::TypeAlias
+            | RustdocItemKind::AttributeMacro
+            | RustdocItemKind::DeriveMacro => {
+                path_components
+                    .push(format!("{kind}.{name}.html", kind = self.kind.class()).into());
+            }
+        }
+
+        path_components.join("/")
+    }
+}