add preface

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

mini-lsm-book/src/SUMMARY.md

@@ -1,7 +1,8 @@
 # LSM in a Week
 
-[Overview](./00-overview.md)
-[Get Started](./00-get-started.md)
+[Preface](./00-preface.md)
+[Mini-LSM Overview](./00-overview.md)
+[Environment Setup](./00-get-started.md)
 
 ---
 
@@ -22,7 +23,7 @@
 
 # Mini-LSM v2
 
-- [Week 1: Mini-LSM](./week1-overview.md)
+- [Week 1 Overview: Mini-LSM](./week1-overview.md)
   - [Memtable](./week1-01-memtable.md)
   - [Merge Iterator](./week1-02-merge-iterator.md)
   - [Block](./week1-03-block.md)
@@ -31,7 +32,7 @@
   - [Write Path](./week1-06-write-path.md)
   - [Snack Time: SST Optimizations](./week1-07-sst-optimizations.md)
 
-- [Week 2: Compaction and Persistence](./week2-overview.md)
+- [Week 2 Overview: Compaction and Persistence](./week2-overview.md)
   - [Compaction Implementation](./week2-01-compaction.md)
   - [Simple Compaction Strategy](./week2-02-simple.md)
   - [Tiered Compaction Strategy](./week2-03-tiered.md)
@@ -40,7 +41,7 @@
   - [Write-Ahead Log (WAL)](./week2-06-wal.md)
   - [Snack Time: Batch Write and Checksums](./week2-07-snacks.md)
 
-- [Week 3: MVCC](./week3-overview.md)
+- [Week 3 Overview: MVCC](./week3-overview.md)
 
 # The Rest of Your Life (TBD)