Add a sample bash script to docs for creating multiple worker files (#13032)

Signed-off-by: Ville Petteri Huh.

changelog.d/13032.doc

+Add a helpful example bash script to the contrib directory for creating multiple worker configuration files of the same type. Contributed by @villepeh.

contrib/workers-bash-scripts/create-multiple-workers.md

+# Creating multiple workers with a bash script
+
+Setting up multiple worker configuration files manually can be time-consuming.
+You can alternatively create multiple worker configuration files with a simple `bash` script. For example:
+
+```sh
+#!/bin/bash
+for i in {1..5}
+do
+cat << EOF >> generic_worker$i.yaml
+worker_app: synapse.app.generic_worker
+worker_name: generic_worker$i
+
+# The replication listener on the main synapse process.
+worker_replication_host: 127.0.0.1
+worker_replication_http_port: 9093
+
+worker_listeners:
+  - type: http
+    port: 808$i
+    resources:
+      - names: [client, federation]
+
+worker_log_config: /etc/matrix-synapse/generic-worker-log.yaml
+EOF
+done
+```
+
+This would create five generic workers with a unique `worker_name` field in each file and listening on ports 8081-8085.
+
+Customise the script to your needs.