message: docs: Update testnet/mainnet URLs
diff: # Mainnet and Testnet Configuration
  -## Mainnet
  +## Mainnet (https://network.thegraph.com)
  ### Registration / Funding (GRT)
  @@ -62,7 +62,7 @@ indexer. The only requirement is a minimum stake of 100k GRT.
  | `ethereum` | `--ethereum-rpc` | `mainnet:...` |
  | `ipfs` | `--ipfs` | `https://ipfs.network.thegraph.com` |
  -## Testnet
  +## Testnet (https://testnet.thegraph.com/)
  ### Registration / Funding (GRT)
  @@ -104,14 +104,14 @@ testnet (for now) are all mainnet subgraphs. This means:
  #### Indexer Agent
  | Environment Variable | CLI Argument | Value |
  -| ----------------------------------------- | ----------------------------- | --------------------------------------------------- |
  +| ----------------------------------------- | ----------------------------- | ------------------------------------------------- |
  | `INDEXER_AGENT_ETHEREUM` | `--ethereum` | An Ethereum rinkeby node/provider |
  | `INDEXER_AGENT_ETHEREUM_NETWORK` | `--ethereum-network` | `4` (rinkeby) |
  | `INDEXER_AGENT_INDEXER_ADDRESS` | `--indexer-address` | Ethereum address of testnet indexer |
  | `INDEXER_AGENT_INDEXER_GEO_COORDINATES` | `--indexer-geo-coordinates` | Geo coordinates of testnet indexer infrastructure |
  | `INDEXER_AGENT_MNEMONIC` | `--mnemonic` | Ethereum mnemonic for testnet operator |
  | `INDEXER_AGENT_NETWORK_SUBGRAPH_ENDPOINT` | `--network-subgraph-endpoint` | `https://gateway.testnet.thegraph.com/network` |
  -| `INDEXER_AGENT_DAI_CONTRACT` | `--dai-contract` | `0xa0b86991c6218b36c1d19d4a2e9eb0ce3606eb48` (USDC) |
  +| `INDEXER_AGENT_DAI_CONTRACT` | `--dai-contract` | `?` (GDAI?) |
  #### Indexer Service
  