Allow to specify a sub menu

[Baptouuuu]

Need

The original ReadTheDocs template supports sub menus in the sidebar. This template should also support this feature.

Implementation

• Allow to specify items in a top level item, with sub items having the same structure as the top level ones (keys: text, relativeUrl and absoluteUrl
• Sub menus are only displayed for the currently opened top level menu
• When a top (non active) level item has a sub menu a little + symbol is displayed on the left (to invite people to go discover the sub menu)

Notes

• The css comes from the original RTD template (though a copy/paste via the web inspector)
  • the css is in theme-fixes.css as this file doesn't seem to be generated and I don't know to generate the theme.css from the sass files
• The + symbol is originally always present with the capability to open/close on click. I didn't implement this feature to keep things (and js file) simple
• The current implementation should not break existing projects

[mnapoli]

Thanks!