Make the editor responsive

Currently the editor is designed for desktop screens only. To allow for different sizes, here are some suggestions:

Collapse the menu from least to most accessed groups.
Shrink and eventually remove the margin of the page on smaller screen sizes.
Shrink the margin of the content on smaller screen sizes.
Stick the style menu to the bottom of the viewport on touch screen and mobile devices.

jlvandenhout / docusaurus-plugin-docs-editor