Show the dashboard on the Docker Hub page

The page described the application in three paragraphs and then went
straight to a compose file. Somebody deciding whether to pull it had no
idea what it looks like — and for a thing whose whole job is a screen your
clients use, that is the question they are actually asking.

The dashboard, after the paragraphs that say what this is and before the
quick start, which is the point in the page where a reader has decided they
are interested and not yet decided to spend ten minutes.

The same image the README uses, and the same alt text, which was written to
describe the screen rather than to name the file. One screenshot, not
three: the README has the other two and the caption says so, and a registry
description that scrolls past its own install instructions has stopped
being an install page.

Absolute raw.githubusercontent URL, because a repository-relative path
resolves to nothing on hub.docker.com — the same reason the badges point
there. .github/ is stripped from the release artifact, which does not
matter here: this file is pasted into a description, not shipped.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docker/production/dockerhub-overview.md

@@ -20,6 +20,11 @@ per-seat pricing. It runs on your server, and the files stay there.
 
 This is the official image for the Community edition — free software under the GPL v2 or later.
 
+![The dashboard: counters for files, clients and groups, the clients using the most storage against their quotas, a month of uploads and downloads as a line chart, and recent activity](https://raw.githubusercontent.com/projectsend/projectsend/main/.github/screenshots/dashboard.png)
+
+*The dashboard — what is in the installation, and what has been happening in it. More screenshots
+are in the [README](https://github.com/projectsend/projectsend#screenshots).*
+
 ---
 
 ## Quick start