docs: add v0.8.0 highlights (#972)

Co-authored-by: Yiran <cuiyiran3@gmail.com>

docs/release-notes/release-0-8-0.md

@@ -1,6 +1,15 @@
 # v0.8.0
 
-Release date: May 17, 2024
+Release date: May 18, 2024
+
+## 👍 Highlights
+
+- [Flow Engine](https://docs.greptime.com/user-guide/continuous-aggregation/overview): which brings continuous aggregation capabilities.
+- [Column Type Modification](https://docs.greptime.com/reference/sql/alter#modify-column-type): allows you to effortlessly alter the data type of columns within a table without the hassle of rebuilding the table or manually migrating data.
+- [Cluster Management Information Table](https://docs.greptime.com/reference/sql/information-schema/overview): allows querying for information about the cluster. This functionality aids administrators in monitoring and managing the health status of the database cluster, facilitating prompt issue detection and resolution.
+- [Append-only Tables](https://docs.greptime.com/reference/sql/create#table-options): You can now create tables in Append-only mode by setting the append mode during table creation.
+- [DROP DATABASE](https://docs.greptime.com/reference/sql/drop.md#drop-database): enables swift deletion of all tables and resources under a database instance.
+- [`EXPLAIN ANALYZE <QUERY>`](https://docs.greptime.com/reference/sql/explain.md): analyze and optimize query statements swiftly in distributed mode.
 
 ## Breaking changes