Adds documentation about setting alternate local metadata/config path

docs/pages/guides/configuration.mdx

@@ -6,6 +6,8 @@ import { Callout } from "nextra/components";
 
 The `config/config.json` file configures all steps of the pipeline. You can use the `config/config.template.json` file as a template to create your own configuration file.
 
+For setting up alternate locations for the `config.json` file, see the [metadata section](/guides/metadata)
+
 <Callout type="info" emoji="💡">
 
 The full schema specification and description of each parameter can be found [in the API Reference section](/api-reference/configuration).

docs/pages/guides/metadata.mdx

@@ -18,6 +18,20 @@ To configure the pipeline with the metadata, you have two options: Save them loc
 
 For this option, you can save the three files `locations.json`, `sensors.json`, and `campaigns.json` to the `config/` directory of the pipeline directory. Be sure to include all the files - even if you don't have any campaigns, save an empty list to `campaigns.json`.
 
+Alternatively, you can store these files locally in a location outside of this repository. This is achieved by setting the environment variable `ERP_CONFIG_DIR` to the full path of the alternate location (ERP <- EM27 Retrieval Pipeline). Note that this should be the same directory that contains the `config.json` file. If this environment variable is not set, the default location the pipeline will look for these files is `./config/`. When setting an alternate config directory, leave `./config/config.template.json` for tests (this file is version-controlled).
+
+For example, directly setting the environment variable:
+```
+export ERP_CONFIG_DIR=<path-to-local-config-dir>
+```
+
+For example, sourcing a `.env` file:
+```
+set -a && . <path-to-.env-file> && set +a
+```
+
+Check the set environment variables with `export -p`.
+
 ### Option 2: GitHub Repository
 
 **1. Create a repository**