Add donation banner to documentation.

docs/_templates/layout.html

 {% extends "!layout.html" %}
 
+{% block relbar1 %}
+{{ super() }}
+{% if pagename != "src/donating" %}
+<div style="width: 100%; height: 3em; vertical-align: middle; text-align: center; font-weight: bold; background-color: lightgray; border: solid red 1px">
+    <p style="font-size: 110%">Like the tool? Help making it better! <a href="{{ pathto("src/donating") }}">Your donation helps!</a></p>
+</div>
+{% endif %}
+{% endblock %}
+
+
 {% block footer %}
 {{ super() }}
 {#  ## Removed to avoid tracking our users.

docs/src/donating.rst

+🌷️ Thank you for your interest in supporting Cython! 🌷️
+======================================================
+
+Managing, maintaining and advancing a project as large as Cython takes
+**a lot of time and dedication**.  This is really a full-time job that
+is currently done by **Stefan Behnel**, paid by users just like you.
+
+**Your support can make a difference**
+for improving a great tool that helps you every day!
+
+Please consider signing a subscription for continuous project support via
+
+* `GitHub Sponsors <https://github.com/users/scoder/sponsorship>`_
+* `Tidelift <https://tidelift.com/subscription/pkg/pypi-cython>`_
+* `PayPal <https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=HLS9JEYD4ETB6&source=url>`_
+
+or donating via
+
+* `PayPal <https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=HLS9JEYD4ETB6&source=url>`_