Update index.md (MystenLabs#785)

Creating Build docs landing page with links to and summaries of subpages.

doc/src/build/index.md

@@ -1,13 +1,18 @@
 ---
-title: Build
+title: Building Sui
 ---
 
-## Key Concepts
-- [Authorities](authorities.md)
-- [Objects](objects.md)
-- [Transactions](transactions.md)
-- TODO: Programmability
-- TODO: clients
-- TODO: state sync
-- TODO: reconfiguration
-- TODO governance, incentives, and rewards
+Now that you've [learned about Sui](../learn/index.md), it's time to build it yourself. Start by becoming familiar with these key concepts:
+
+* [Smart Contracts with Move](move.md) - Move is an open source language for writing safe smart contracts. In Sui, Move is used to define,
+  create and manage programmable Sui objects representing user-level assets.
+* [Sui Wallet](wallet.md) - Sui wallet was developed to facilitate local experimentation with Sui features. In this document, we describe
+  how to set up Sui wallet and execute wallet commands through its command line interface, Wallet CLI.
+* [Authorities](authorities.md) - The Sui network is operated by a set of independent authorities, each running its own instance of the Sui
+  software on a separate machine (or a sharded cluster of machines operated by the same entity).
+* [SuiJSON](sui-json.md) - SuiJSON is a JSON-based format with restrictions that allow Sui to align JSON inputs more closely with Move Call
+  arguments. This table shows the restrictions placed on JSON types to make them SuiJSON compatible.
+* [Objects](objects.md) - Sui has programmable objects created and managed by Move packages (a.k.a. smart contracts). Move packages themselves
+  are also objects. Thus, Sui objects can be partitioned into two categories mutable data values and immutable packages.
+* [Transactions](transactions.md) - All updates to the Sui ledger happen via a transaction. This section describes the transaction types
+  supported by Sui and explains how their execution changes the ledger.