docs: add beacon configuration file (rcConfig) reference

docs/pages/run/beacon-management/configuration-file.md

@@ -0,0 +1,76 @@
+---
+title: Configuration File
+---
+
+# Configuration File
+
+Lodestar can read any command line option from a configuration file using the global `--rcConfig` flag. This is useful for managing larger setups, keeping your configuration under version control, or avoiding long commands with many flags.
+
+`--rcConfig` is available for all commands (`beacon`, `validator`, `bootnode`, etc.). The accepted file formats are YAML (`.yml`, `.yaml`) and JSON (`.json`).
+
+## File Format
+
+Each key maps to a CLI flag, written without the leading `--`, and the value uses the same type as the flag (string, number, boolean, or array). Nested options such as `--rest.address` are written as dotted keys (`rest.address`), not as nested maps.
+
+For the full list of available options, see the [`beacon`](./beacon-cli.md), [`validator`](../validator-management/validator-cli.md), and [`bootnode`](../bootnode/bootnode-cli.md) CLI references.
+
+```yaml
+# beacon.config.yaml
+network: "hoodi"
+dataDir: "/data/lodestar"
+logFile: "/data/lodestar/beacon.log"
+jwtSecret: "/data/jwtsecret.hex"
+
+# Networking
+listenAddress: "0.0.0.0"
+port: 9000
+
+# Beacon REST API
+rest: true
+rest.address: "0.0.0.0"
+rest.port: 9596
+
+# Execution client
+execution.urls:
+  - "http://localhost:8551"
+
+# Metrics
+metrics: true
+metrics.port: 8008
+```
+
+The same configuration in JSON:
+
+```json
+{
+  "network": "hoodi",
+  "dataDir": "/data/lodestar",
+  "logFile": "/data/lodestar/beacon.log",
+  "jwtSecret": "/data/jwtsecret.hex",
+  "listenAddress": "0.0.0.0",
+  "port": 9000,
+  "rest": true,
+  "rest.address": "0.0.0.0",
+  "rest.port": 9596,
+  "execution.urls": ["http://localhost:8551"],
+  "metrics": true,
+  "metrics.port": 8008
+}
+```
+
+## Running with a Configuration File
+
+Pass the file path to `--rcConfig` when starting the node:
+
+```bash
+./lodestar beacon --rcConfig /data/beacon.config.yaml
+```
+
+## Overriding Options
+
+Options passed directly on the command line take precedence over the values in the configuration file. This lets you keep a shared base configuration and override individual values per run:
+
+```bash
+# Use everything from the file, but connect to mainnet instead of hoodi
+./lodestar beacon --rcConfig /data/beacon.config.yaml --network mainnet
+```

docs/pages/run/beacon-management/starting-a-node.md

@@ -16,7 +16,7 @@ Make sure Lodestar is installed in your local environment, following the chosen
 
 For a complete list of beacon node CLI commands and options, see the [`beacon` CLI Command](../beacon-management/beacon-cli.md) section.
 
-To select a known testnet or mainnet, use the `--network` flag. The option `mainnet` is selected by default, and a list of available networks is listed with the `--help` flag. Setting the `--network` flag will conveniently configure the beacon node for the selected network. For power users, any configuration option should be able to be overridden.
+To select a known testnet or mainnet, use the `--network` flag. The option `mainnet` is selected by default, and a list of available networks is listed with the `--help` flag. Setting the `--network` flag will conveniently configure the beacon node for the selected network. For power users, any configuration option should be able to be overridden. You can also supply options through a [configuration file](./configuration-file.md) instead of passing them as command line flags.
 
 ## Configure the Lodestar JWT Authentication Token
 

docs/sidebars.ts

@@ -24,6 +24,7 @@ const sidebars: SidebarsConfig = {
           label: "Beacon Node",
           items: [
             "run/beacon-management/starting-a-node",
+            "run/beacon-management/configuration-file",
             "run/beacon-management/beacon-cli",
             "run/beacon-management/data-retention",
             "run/beacon-management/fast-confirmation",