default to finding tex2svg on user's $PATH

[dankessler]

If tex2svg is installed in such a way that it is available on $PATH (from within emacs), then there's no need for the user to install it manually.

I've set the default value of org-latex-impatient-tex2svg-bin to simply tex2svg (as opposed to the empty string which it was previously set to); this default will work if emacs can find tex2svg on the path. I've also updated the documentation to specify that the user may need to set the value (since otherwise things may just work). I've also adjusted the documentation to suggest a global installation of mathjax-node-cli (and fixed some indentation while I was at it), since IMHO this makes it more likely that tex2svg will be on $PATH (e.g., on MacOS, if node is installed using homebrew, then npm install -g mathjax-node-cli will put tex2svg under /opt/homebrew/bin/tex2svg, which is likely to be on $PATH