Move assets to the docs folder

[xparadoxical]

I don't know why the screenshots are referenced as github links, but relative paths work fine for images and let you preview changes locally properly. When using relative paths with docsify, you can't go deeper than the /docs folder (the one being docsify served), which means /assets would have to be moved to /docs/assets, but this makes sense structurally anyway, as /docs is the website root and these are assets of the documentation. The gddocs-icon.png can be left in /assets in repo root.

I'd also move stylesheets and fonts (being added in #59) to the new location of assets.

Not sure if #56 and #59 have to be merged before doing this to get no merge conflicts so I'll wait.