feat(book): finish overview

Signed-off-by: Alex Chi <iskyzh@gmail.com>

mini-lsm-starter/src/block/builder.rs

@@ -0,0 +1,29 @@
+#![allow(unused_variables)] // TODO(you): remove this lint after implementing this mod
+#![allow(dead_code)] // TODO(you): remove this lint after implementing this mod
+
+use super::Block;
+
+/// Builds a block
+pub struct BlockBuilder {}
+
+impl BlockBuilder {
+    /// Creates a new block builder
+    pub fn new(target_size: usize) -> Self {
+        unimplemented!()
+    }
+
+    /// Adds a key-value pair to the block
+    #[must_use]
+    pub fn add(&mut self, key: &[u8], value: &[u8]) -> bool {
+        unimplemented!()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        unimplemented!()
+    }
+
+    /// Builds a block
+    pub fn build(self) -> Block {
+        unimplemented!()
+    }
+}