Revert "Delete index.mdx"

This reverts commit 4b47c33.

Author: josh-wong

docs/index.mdx

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+# ScalarDB
+
+import { CardRowAbout, CardRowGettingStarted, CardRowSamples, CardRowDevelop, CardRowDeploy, CardRowManage, CardRowReference } from '/src/components/Cards';
+
+ScalarDB is a cross-database HTAP engine. It achieves ACID transactions and real-time analytics across diverse databases to simplify the complexity of managing multiple databases.
+
+**About ScalarDB**
+
+<CardRowAbout />
+
+**Getting started**
+
+<CardRowGettingStarted />
+
+**Samples**
+
+<CardRowSamples />
+
+**Develop**
+
+<CardRowDevelop />
+
+**Deploy**
+
+<CardRowDeploy />
+
+**Manage**
+
+<CardRowManage />
+
+**Reference**
+
+<CardRowReference />