[#551] feat(doc): Add overview doc (#558)

### What changes were proposed in this pull request?

This PR adds an overview doc to explain Gravitino at a high level.

### Why are the changes needed?

Fix: #551 

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

No

docs/overview.md

@@ -0,0 +1,43 @@
+---
+title: "Overview"
+date: 2023-10-19T15:33:00-08:00
+license: "Copyright 2023 Datastrato.
+This software is licensed under the Apache License version 2."
+---
+
+# Overview
+
+## Introduction
+
+Gravitino is a high-performance, geo-distributed, and federated metadata lake. It manages the metadata directly in different sources, types, and regions. It also provides users unified access to the metadata for both data and AI assets.
+
+![Gravitino Architecture](assets/gravitino-architecture.png)
+
+Gravitino aims to provide several key features:
+
+* SSOT (Single Source of Truth) for multi-regional data with geo-distributed architecture support.
+* Unified Data + AI assets managements for both users and engines.
+* Security in one place, centralize the security for different sources.
+* Built-in data management + data access management.
+
+## Architecture
+
+![Gravitino Model and Arch](assets/gravitino-model-arch.png)
+
+* **Functionality Layer**: Gravitino provides a set of APIs for users to manage and govern the 
+  metadata, including standard metadata creation, update, and delete operations. In the meantime, it also provides the ability to govern the metadata in a unified way, including access control, discovery, and others.
+* **Interface Layer**: Gravitino provides standard REST APIs as the interface layer for users. It will also provide Thrift and JDBC interfaces in the future. 
+* **Core Object Model**: Gravitino defines a generic metadata model to represent the metadata in different sources and types, manages them in a unified way.
+* **Connection Layer**: In the connection layer, Gravitino provides a set of connectors to connect to different metadata sources, including Hive, MySQL, PostgreSQL, and others. It also provides the ability to connect and manage heterogeneous metadata other than Tabular data.
+
+## Terminology
+
+The model of Gravitino
+
+![Gravitino Model](assets/metadata-model.png)
+
+* **Metalake**: The top-level container for metadata. Typically, one group has one metalake to manage all the metadata in it. Each metalake exposes a three-level namespace(catalog.schema.table) to organize the data.
+* **Catalog**: catalog is a collection of metadata from a specific metadata source. Each catalog will have a related connector to connect to the specific metadata source.
+* **Schema**: Schema is equivalent to database, Schemas only exist in the specific catalogs that support relational metadata sources, such as Hive, MySQL, PostgreSQL, and others.
+* **Table**: The lowest level in the object hierarchy for catalogs that support relational metadata sources. Tables can be created in the specific schemas in the catalogs.
+* **Model**: Model represents the metadata in the specific catalogs that support model management.