chore(docs): refine part 1, 2

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

mini-lsm-starter/src/table.rs

@@ -57,7 +57,11 @@ impl FileObject {
     }
 }
 
-pub struct SsTable {}
+pub struct SsTable {
+    file: FileObject,
+    block_metas: Vec<BlockMeta>,
+    block_meta_offset: usize,
+}
 
 impl SsTable {
     /// Open SSTable from a file.

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

@@ -9,6 +9,7 @@ use super::{BlockMeta, SsTable};
 /// Builds an SSTable from key-value pairs.
 pub struct SsTableBuilder {
     pub(super) meta: Vec<BlockMeta>,
+    // Add other fields you need.
 }
 
 impl SsTableBuilder {