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.rs

@@ -21,6 +21,7 @@ impl Block {
         for offset in &self.offsets {
             buf.put_u16(*offset);
         }
+        // Adds number of elements at the end of the block
         buf.put_u16(offsets_len as u16);
         buf.into()
     }