Update supported-networks.mdx (#384)

Author: camdengrieh

website/pages/en/developing/supported-networks.mdx

@@ -21,6 +21,8 @@ The following networks are supported on The Graph's Decentralized Network:
 The following networks are supported in beta on the Hosted Service:
 
 - `goerli`
+- `arbitrum-goerli`
+- `avalanche-fuji`
 - `poa-core`
 - `poa-sokol`
 - `matic` (now known as Polygon)
@@ -30,6 +32,7 @@ The following networks are supported in beta on the Hosted Service:
 - `bsc` (now known as BNB Chain)
 - `chapel`
 - `clover`
+- `celo-alfajores`
 - `fuji`
 - `fuse`
 - `moonriver`