Add PNG version of stdtolocal image for web rendering of dev guide

In Section 2.2 of the developer guide, figure 2.1, doesn't appear in the web-rendered version. The page is linking to a PNG version of the stdtolocal graphic while only a PDF seems to be in the repo.

Rather than simply change the link to PDF, I think a PNG version of the image is probably more sensible for the web version of the guide anyway so I've created a PNG version of the image and added this to the repo in addition to the existing (vector?) PDF version which I guess may be used for generating the PDF version of the guide.

(Since this MR only adds an image file and a symlink to the repo and doesn't change any existing files, I've skipped the vast majority of pipline jobs, leaving only those related to documentation to run.)