svg images are linked in a way browsers don't like
If overview.svg exists, then
.. image:: _images/overview.* :alt: Overview Diagram
<object data="_images/overview.svg" type="image/svg+xml"><embed src="_images/overview.svg" type="image/svg+xml"></object>
using the HTML builder. In Firefox, this renders as an object needing a plugin (for which no plugin exists). On Chrome, the browser tries to download the images. On Safari, the images are displayed, but inside small boxes with scroll bars.
I think the much simpler <img src="_images/overview.svg"> would work, perhaps with a warning that older browsers do not adequately support SVG, and that bitmap-based formats should be used instead where extreme browser compatibility is important.