Docs: align README badges, drop Gitter, note upstream deprecation

Align the badge block with the rest of the Dockette org (copybara /
apidoc style): four img.shields.io badges (Actions, Docker Hub pulls,
Sponsor, Support/Discussions) with consistent alt text.

Drop the now-dead Gitter badge and the redundant Docker Stars badge.
Add a short upstream-status callout so users know that
GoogleChrome/rendertron is archived and marked deprecated upstream,
and that the image is kept for existing users and the demo
screenshot.

Add the standard Maintenance section to match the rest of the org.

Co-authored-by: Felix <rkfelix@gmail.com>

Author: ohmyfelix

README.md

@@ -2,16 +2,18 @@
 
 Ready-to-use image for SSR rendering based on headless Chrome, [a.k.a Rendertron by Google](https://github.com/GoogleChrome/rendertron).
 
-> All credits belong to authors of GoogleChrome/rendertron.
+> All credits belong to the authors of [GoogleChrome/rendertron](https://github.com/GoogleChrome/rendertron).
 
------
-
-[![Docker Stars](https://img.shields.io/docker/stars/dockette/rendertron.svg?style=flat)](https://hub.docker.com/r/dockette/rendertron/)
-[![Docker Pulls](https://img.shields.io/docker/pulls/dockette/rendertron.svg?style=flat)](https://hub.docker.com/r/dockette/rendertron/)
+> **Upstream status:** the [GoogleChrome/rendertron](https://github.com/GoogleChrome/rendertron) project was archived and the upstream README marks it as deprecated. The image here is kept for existing users and the demo screenshot; new projects should evaluate the alternative rendering strategies linked from the upstream README before adopting Rendertron.
 
-## Discussion / Help
+-----
 
-[![Join the chat](https://img.shields.io/gitter/room/dockette/dockette.svg?style=flat-square)](https://gitter.im/dockette/dockette?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+<p align=center>
+  <a href="https://github.com/dockette/rendertron/actions"><img src="https://github.com/dockette/rendertron/actions/workflows/docker.yml/badge.svg" alt="GitHub Actions"></a>
+  <a href="https://hub.docker.com/r/dockette/rendertron"><img src="https://img.shields.io/docker/pulls/dockette/rendertron.svg" alt="Docker Hub pulls"></a>
+  <a href="https://github.com/sponsors/f3l1x"><img src="https://img.shields.io/badge/sponsor-GitHub%20Sponsors-ea4aaa" alt="GitHub Sponsors"></a>
+  <a href="https://github.com/orgs/dockette/discussions"><img src="https://img.shields.io/badge/support-discussions-6f42c1" alt="Support/Discussions"></a>
+</p>
 
 ## Usage