Write docs on starknet-contract target

Author: mkaput

website/pages/docs/_meta.json

@@ -7,6 +7,9 @@
   "reference": {
     "title": "Reference"
   },
+  "starknet": {
+    "title": "Starknet"
+  },
   "extensions": {
     "title": "Extensions"
   }

website/pages/docs/starknet/contract-target.mdx

@@ -0,0 +1,13 @@
+# Starknet Contract Target
+
+The `starknet-contract` target allows to build the package as a [Starknet Contract](https://docs.starknet.io/documentation/getting_started/intro/).
+It searches for all contract classes in the package, and builds a separate compiled JSON file each found class.
+Generated file will be named with following pattern: `[package name]_[contract name].json`.
+
+To enable it for a package, write a following line in `Scarb.toml`:
+
+```toml
+[[target.starknet-contract]]
+```
+
+This target does not accept any configuration arguments beside ones [shared among all targets](../reference/targets).