Remove artifacts from Emscripten_logo_full, added single logo svg/png (…

…#13790)

Closes: #13783

Fixed site/source/_static/Emscripten_logo_full.png by removing bright artifacts from background masking - that looked not so good on dark backgrounds. Reproduce other minor changes not in the SVG.

Added logo.svg / logo.png that with an intention to update logo of https://github.com/emscripten-core - so that posts linked in social media will look better. Used SVG is a modified version of switch_logo.svg by removing label and adjust page size.