Added Nimbus to the key loader docs

Author: jclapis

docs/docs/get_started/configuration.md

@@ -52,7 +52,7 @@ keys_path = "/path/to/keys"
 secrets_path = "/path/to.secrets"
 ```
 
-We currently support Lighthouse, Prysm, Teku and Lodestar's keystores so it's easier to load the keys. We're working on adding support for additional keystores. These are the expected file structures for each format:
+We currently support Lighthouse, Prysm, Teku, Lodestar, and Nimbus's keystores so it's easier to load the keys. We're working on adding support for additional keystores. These are the expected file structures for each format:
 
 <details>
   <summary>Lighthouse</summary>
@@ -175,6 +175,37 @@ We currently support Lighthouse, Prysm, Teku and Lodestar's keystores so it's ea
   :::
 </details>
 
+<details>
+  <summary>Nimbus</summary>
+
+  #### File structure:
+  ```
+  ├── keys
+  │   ├── <PUBLIC_KEY_1>
+  │   │   └── keystore.json
+  │   └── <PUBLIC_KEY_2>
+  │       └── keystore.json
+  └── secrets
+      ├── <PUBLIC_KEY_1>
+      └── <PUBLIC_KEY_2>
+  ```
+
+  #### Config:
+  ```toml
+  [pbs]
+  ...
+  with_signer = true
+
+  [signer]
+  port = 20000
+
+  [signer.local.loader]
+  format = "nimbus"
+  keys_path = "keys"
+  secrets_path = "secrets"
+  ```
+</details>
+
 ### Proxy keys store
 
 Proxy keys can be used to sign transactions with a different key than the one used to sign the block. Proxy keys are generated by the Signer module and authorized by the validator key. Each module have their own proxy keys, that can be BLS or ECDSA.