Add external verification documentation page (#47)

* Add external verification documentation page

* Update docs/3. Verification/11. externalVerifiers.md

Co-authored-by: Kaan Uzdoğan <kaanuzdogan@gmail.com>

---------

Co-authored-by: Kaan Uzdoğan <kaanuzdogan@gmail.com>

Author: marcocastignoli

docs/3. Verification/11. externalVerifiers.md

@@ -0,0 +1,62 @@
+---
+id: external-verifiers
+title: Automatic verification on external verifiers
+slug: /external-verifiers
+---
+
+# Automatic verification on external verifiers
+
+Whenever Sourcify finds a match, it automatically forwards the contract for verification to external verification services like Blockscout, Routescan, and Etherscan. In practice, this lets Sourcify act as a single verification entry point, so a contract gets verified across all supported explorers in one go.
+
+Keep in mind Etherscan API has a strict 200 verifications/day per API key. This limit gets filled quickly and we can't share all verifications with Etherscan.
+
+Sourcify can trigger external verification through any of its API endpoints, but tracking the verification's status is only supported by the v2 APIs. Now the verification job endpoint (`/v2/verify/{verificationId}`) returns the external status URL you can poll. The response also includes a direct link to the contract’s page on the relevant explorer
+
+```json
+{
+    ...,
+    externalVerifications: {
+        blockscout: {
+            verificationId: "x3ryqcqr1zdknhfhkimqmizlcqpxncqc6nrvp3pgrcpfsqedqi",
+            statusUrl: "https://api.etherscan.io/v2/api?chainid=1&module=contract&action=checkverifystatus&guid=x3ryqcqr1zdknhfhkimqmizlcqpxncqc6nrvp3pgrcpfsqedqi",
+            explorerUrl: "https://etherscan.io/address/0x0D41E0B7bbdEBDBc684020dcb64ADe76C0338222#code",
+        }
+    }
+    ...
+}
+```
+
+Via the Sourcify UI users can keep track of the external verifications' status live.
+
+## Enabling external verification in Sourcify Server
+
+If you are maintaining a Sourcify server instance you can enable external verification by using the appropriate Storage services. The appropriate storage service must be configured for each external verifier.
+
+```json
+// services/server/src/config/local.js
+{
+  ...
+  storage: {
+    ...
+    writeOrWarn: [
+      ...
+      WStorageIdentifiers.EtherscanVerify,
+      WStorageIdentifiers.BlockscoutVerify,
+      WStorageIdentifiers.RoutescanVerify,
+      ...
+    ],
+    ...
+  },
+  ...
+}
+```
+
+Remember also to specify the API keys.
+
+```bash
+# services/server/.env
+BLOCKSCOUT_API_KEY=
+ROUTESCAN_API_KEY=
+# warn: Etherscan have a very strict rate limit of 200 verifications / day.
+ETHERSCAN_API_KEY= 
+```