Revert "removed duplicate file"

This reverts commit 025b48af3f.

src/pages/index.md

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+---
+title: Stacks documentation
+description: Learn about Stacks mining, the STX token, and the Clarity smart contract language
+---
+
+-> Content related to developer tools and app development has recently moved to [docs.hiro.so](https://docs.hiro.so/). For more information on the content move, see [this post](https://forum.stacks.org/t/the-evolution-of-the-stacks-documentation-and-a-new-hiro-docs-site/12343) on the Stacks forum.
+
+## Understand Stacks
+
+[@page-reference | grid]
+| /understand-stacks/overview, /understand-stacks/proof-of-transfer, /understand-stacks/testnet
+
+## Write smart contracts
+
+[@page-reference | grid]
+| /write-smart-contracts/overview, /write-smart-contracts/tokens
+
+## Start mining
+
+[@page-reference | grid]
+| /start-mining/mainnet, /start-mining/testnet