usingplanttracerwebapp.html: modify to use the webapp

[sbarber2]

Right now usingplanttracerwebapp.html is a copy of usingplanttracerios.html.

It needs to be modified to use the webapp rather than the iOS app and then linked in to to the static site flow.

Remember to redo the instructions for both desktop and mobile.

[sbarber2]

Remember to re-enable the rendering of screenshots for usingplanttracerwebapp.html in render_pages.py