docs: add comments & hints for day one starter and reference code (#18)

* feat(docs): Improve/Add comments & some hints for day one starter code

* feat(docs): Add comments for day one solution code

* feat(docs): Add figure for block storage format in starter code (block.rs)

mini-lsm/src/block/builder.rs

@@ -30,11 +30,13 @@ impl BlockBuilder {
     #[must_use]
     pub fn add(&mut self, key: &[u8], value: &[u8]) -> bool {
         assert!(!key.is_empty(), "key must not be empty");
+        // The overhead here is `key_len` + `val_len` + `offset`, each is of type `u16`
         if self.estimated_size() + key.len() + value.len() + SIZEOF_U16 * 3 > self.block_size
             && !self.is_empty()
         {
             return false;
         }
+        // The offsets should be updated at first, to maintain the correct offset
         self.offsets.push(self.data.len() as u16);
         self.data.put_u16(key.len() as u16);
         self.data.put(key);