Adding Stellar (#914)

Adding link

Author: idalithb

website/src/pages/en/sps/introduction.mdx

@@ -28,3 +28,4 @@ Visit the following links for tutorials on using code-generation tooling to buil
 - [Starknet](https://docs.substreams.dev/tutorials/intro-to-tutorials/starknet)
 - [Injective](https://docs.substreams.dev/tutorials/intro-to-tutorials/on-cosmos/injective)
 - [MANTRA](https://docs.substreams.dev/tutorials/intro-to-tutorials/on-cosmos/mantra)
+- [Stellar](https://docs.substreams.dev/tutorials/intro-to-tutorials/stellar)