theme/css/general.css: emphasize active sidebar item

[ethamck]

May (help) close https://github.com/void-linux/void-docs/issues/803, appends a bullet character (U+2022, •) to the end of a selected item in the sidebar for visibility.

For screen readers, this should render as "selected" instead.

(I would've tried to color the background differently but I'm not a CSS wizard and this seemed more practical.)