Update docs/configuring-playbook-synapse-usage-exporter.md: add the section "What does it do?"

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/configuring-playbook-synapse-usage-exporter.md

@@ -6,6 +6,10 @@ It allows you to export the usage statistics of a Synapse homeserver to this con
 
 Synapse does not include usage statistics in its Prometheus metrics. They can be reported to an HTTP `PUT` endpoint 5 minutes after startup and from then on at a fixed interval of once every three hours. This role integrates a simple [Flask](https://flask.palletsprojects.com) project that offers an HTTP `PUT` endpoint and holds the most recent received record available to be scraped by Prometheus.
 
+See the project's [documentation](https://github.com/loelkes/synapse-usage-exporter/blob/main/README.md) to learn what it does and why it might be useful to you.
+
+## What does it do?
+
 Enabling this service will automatically:
 
 - install the synapse-usage-exporter service
@@ -13,8 +17,6 @@ Enabling this service will automatically:
 - re-configure [Prometheus](./configuring-playbook-prometheus-grafana.md) (if Prometheus is enabled), to periodically scrape metrics from synapse-usage-exporter
 - add a new [Grafana](./configuring-playbook-prometheus-grafana.md) dashboard (if Grafana is enabled) containing Synapse usage statistics
 
-See the project's [documentation](https://github.com/loelkes/synapse-usage-exporter/blob/main/README.md) to learn what it does and why it might be useful to you.
-
 ## Adjusting DNS records (optional)
 
 By default, this playbook installs synapse-usage-exporter on the `matrix.` subdomain, at the `/report-usage-stats/push` path (https://matrix.example.com/report-usage-stats/push). This makes it easy to install it, because it **doesn't require additional DNS records to be set up**. If that's okay, you can skip this section.