Improve build performance and usability of the Icons documentation page

[ao5357]

I have not benchmarked it yet, but it appears the file operations to open and include all the SVG icons directly on the page more than doubles the site's overall build time.

An improved icons documentation page should reduce the Jekyll build time substantially, and should make it just as easy if not easier to find an icon and then copy its liquid embed code. Lazy/indirect SVG loading and JavaScript (conditional?) are valid methods of seeing this improvement in the codebase.