Add deployment docs

docs/deployment.md

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+# Deployment
+
+This page explains how to generate and upload a complete set of metadata and data, which conforms to the [output specification](output_structure.md), to Azure blob storage.
+
+## Overview
+
+The deployment process is divided into two main steps:
+
+1. **Fetching the data.** This means running the entire job for each country. This step does not publish anything to Azure; instead, it generates pickled data that is stored inside `$DAGSTER_HOME`.
+
+2. **Uploading the data.** This means running each of the cloud sensor assets, which have the names `publish...`. There are four of these assets: one for the `countries.txt` file, one for the metadata structs, one for the geometries, and one for the metrics. These assets are tied to custom IO managers which read the pickled data from `$DAGSTER_HOME` and upload it to Azure blob storage.
+
+Both of these steps are automated using the `popgetter.run` module, which is in turn invoked by the `deploy.sh` script in the repository root.
+
+## Required environment variables
+
+The deployment process requires the following environment variables to be set:
+
+ - `$POPGETTER_COUNTRIES`: A comma-separated list of country IDs to generate data for. This list also feeds into the `countries.txt` file.
+
+ - `$ENV`: Set this to `prod` to deploy to Azure. (You can set it to `dev` too, but this will publish the data to a local temporary directory, so it's only useful for testing the script.)
+
+ - `$SAS_TOKEN`: The SAS token for the Azure blob storage account. Contact a popgetter maintainer if you need this.
+
+So, a typical deployment command might look like this:
+
+```bash
+POPGETTER_COUNTRIES=bel,gb_nir ENV=prod SAS_TOKEN="..." ./deploy.sh
+```
+
+Note that the `SAS_TOKEN` value must be quoted, because it contains ampersands which the shell will take to mean "run this command in the background".
+
+## Where are the data stored?
+
+The data and metadata will be uploaded to the ['popgetter' Azure storage account](https://portal.azure.com/#@turing.ac.uk/resource/subscriptions/06e7b12a-f395-4021-9fa2-5305fa01903e/resourceGroups/popgetter/providers/Microsoft.Storage/storageAccounts/popgetter/containersList) under the Urban Analytics Technology Platform subscription: specifically, it will be placed in the container named `prod`, and the directory corresponding to the current version of popgetter.