tpl/images: Add images.QR function

[jmooring]

Closes #13205

Note that this uses (create.Client).FromString under the covers, which seemed a lot simpler than creating a FromBytes or FromImage method.

Site and theme authors should test the rendered image per the documentation:

Always test the rendered QR code after resizing, both on-screen and in print.

[jmooring]

I'll handle styling of the example images on the docs site, so if you build the docs from this repo the examples will be stretched.