Docs: change sidebar collapse controls to actual button elements (#31098)

Docs: change sidebar collapse controls to actual button elements (#31098)

apart from the visual styling, there's probably no good reason why these shouldn't be `<button>` elements, semantically

the buttons still look a shade too button-like, despite using `.btn-light`. the last remnant of button styling can probably be suppressed further, *or* this makes the case for expanding the core button styles to have one that looks completely non-button and non-link like (for cases where an additional style/hint was given already, or it's clear from context that something's an actionable button or link)

Co-authored-by: XhmikosR <xhmikosr@gmail.com>