implement custom links

crates/treehouse/src/tree/attributes.rs

@@ -0,0 +1,40 @@
+use serde::Deserialize;
+
+/// Branch attributes.
+#[derive(Debug, Default, Clone, PartialEq, Eq, Deserialize)]
+pub struct Attributes {
+    /// Unique identifier of the branch.
+    ///
+    /// Note that this must be unique to the _entire_ site, not just a single tree.
+    /// This is because trees may be embedded within each other using [`Content::Link`].
+    #[serde(default)]
+    pub id: String,
+
+    /// Controls how the block should be presented.
+    #[serde(default)]
+    pub content: Content,
+}
+
+/// Controls for block content presentation.
+#[derive(Debug, Default, Clone, PartialEq, Eq, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum Content {
+    /// Children are stored inline in the block. Nothing special happens.
+    #[default]
+    Inline,
+
+    /// Link to another tree.
+    ///
+    /// When JavaScript is enabled, the tree's roots will be embedded inline into the branch and
+    /// loaded lazily.
+    ///
+    /// Without JavaScript, the tree will be linked with an `<a>` element.
+    ///
+    /// The string provided as an argument is relative to the `content` root and should not contain
+    /// any file extensions. For example, to link to `content/my-article.tree`,
+    /// use `content.link = "my-article"`.
+    ///
+    /// Note that `Link` branches must not contain any children. If a `Link` branch does contain
+    /// children, an `attribute`-type error is raised.
+    Link(String),
+}