Update website sidebar to be better use-able

[hasezoey]

Summary

This PR (mainly) updates the documentation side-bar to be better use-able, things include:

• replace span hamburger menu-hack with a svg
• change the container element for #menu > .pure-menu to be a nav instead of implicit div
• change sub-lists to be their own lists (plus css changes necessary)
• show indications of what element in the side-bar is currently hovered over (and use the same effect for the current page)
• change the mobile-menu (logo) header and sidebar to be sticky / fixed, so it can be accessed at any scroll level

re #13314

Screenshots

old active & hover:

new active & hover:

old mobile + scroll:

new mobile + scroll:

PS: these changes should make it easier to change the lists (secondary & tertiary lists) to be made collapsible

PPS: these changes also work great with things like dark-reader

[IslandRhythms]

LGTM, thank you

[vkarpov15]

Thanks 👍