v5: Adjust docs sidebar and navbar (#31119)

* Hide the docs nav on mobile, redesign it for two columns on narrow devices

* Tweak spacing on ToC

* Redesign sidebar and subnav

* Revamp subnav mobile layout so everything's on one line

* Tighten up masthead a little

* Switch the icon for the subnav and do a little icon toggling

* Fix mobile overflow since we changed guters stuff

* Add the widths

* make the subnav icon purple

site/layouts/partials/docs-sidebar.html

@@ -2,7 +2,7 @@
   {{- $url := split .Permalink "/" -}}
   {{- $page_slug := index $url (sub (len $url) 2) -}}
 
-  <ul class="list-unstyled mb-0 pt-1 pb-3">
+  <ul class="list-unstyled mb-0 py-3 pt-md-1">
   {{- range $group := .Site.Data.sidebar -}}
     {{- $link := $group.title -}}
     {{- $link_slug := $link | urlize -}}