Add docs for backfilling session context hash

Author: JacksonMaxfield

{{ cookiecutter.hosting_github_repo_name }}/admin-docs/running-extra-scripts.md

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+# Running Extra Scripts
+
+### Background
+
+While we hope for it to be rare, when we ship changes to `cdp-backend` or `cdp-frontend`
+require changes to the data or file storage system that we cannot make during a normal
+pipeline run, or if you want to manage your data in some way, there is a GitHub Action
+that allows the passthrough of any command straight to the running GitHub Action
+worker.
+
+## Backfilling Session Context Hash
+
+In our [v3.0.4 release of `cdp-backend`](https://github.com/CouncilDataProject/cdp-backend/releases/tag/v3.0.4)
+we added a property to the `Session` collection that stores the `session_context_hash`.
+This value is incredibly useful for programmatic work but isn't _needed_ for the
+web application. As such, if you don't want to run this script, you don't need to,
+but it is easy enough to run.
+
+Open up the [Run Command]({{ cookiecutter.hosting_github_url }}/actions/workflows/run-script.yml)
+GitHub Action and click the "Run workflow" button.
+
+Paste in the following: `add_content_hash_to_sessions --google_credentials_file google-creds.json`
+
+Click the "Run workflow" button and the backfill process should kick off.
+(If the run doesn't appear, try refreshing the page.)
+
+If something goes wrong, please create a
+[GitHub Issue in `cdp-backend`](https://github.com/CouncilDataProject/cdp-backend/issues)