better table legend for proton vs enterprise (#539)

Author: chenziliang

docs/proton-oss-vs-enterprise.md

@@ -4,7 +4,7 @@ Timeplus Proton powers unified streaming and data processing on a single node. I
 
 Note: In the Enterprise column, a leading `+` marks capabilities that are additions beyond Proton.
 
-|          Features                     | **Timeplus Proton**                                                                                                                                                                    | **Timeplus Enterprise**                                                                                                                                                                                                          |
+|          **Categories**                     | **Timeplus Proton**                                                                                                                                                                    | **Timeplus Enterprise**                                                                                                                                                                                                          |
 | ----------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | **Deployment**                | <ul><li>Single-node Docker image</li><li>Single binary on Mac/Linux</li><li>Kubernetes Helm Chart (WIP)</li></ul>                                                                                                          | <ul><li>+ [Cluster](/cluster)</li><li>[Kubernetes Helm Chart](/k8s-helm)</li></ul>                                                                               |
 | **Operation**                   |  <ul><li>[RBAC](/rbac)</li><li>[Tiered Storage](/append-stream-tiered-storage)</li><li>[Materialized View Monitoring](/materialized-view-monitoring)</li><li>[System Monitoring](/system-stream-state-log)</li></ul>                                                                                                                     | <ul><li>+ Timeplus Console for Metrics Visualization</li></ul>                                                                                                                                                      |