content/static,internal/fetch/dochtml: expand all nav elements by default

We’ve received feedback that a fully-collapsed side nav prevents users
from being able to see package structure at a glance. Expand all
functions and types by default.

Also updates CSS for the side nav to not show a scroll bar if the
content doesn’t overflow.

Updates golang/go#40612
Fixes golang/go#40715

Change-Id: I22049e394b8705316e1ebe1cb9691c6317c61c5b
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/248183
Run-TryBot: Andrew Bonventre <[email protected]>
TryBot-Result: kokoro <[email protected]>
Reviewed-by: Jamal Carvalho <[email protected]>

Author: andybons

content/static/css/sidenav.css

@@ -113,7 +113,7 @@
     display: block;
     max-height: calc(100vh - var(--header-height));
     overflow-x: hidden;
-    overflow-y: scroll;
+    overflow-y: auto;
     padding-top: 1rem;
     padding-left: 0.25rem;
     position: sticky;

internal/fetch/dochtml/template.go

@@ -89,7 +89,7 @@ var htmlPackage = template.Must(template.New("package").Funcs(
 
 			{{if .Funcs}}
 				<li class="DocNav-functions" role="none">
-					<span class="DocNav-groupLabel" role="treeitem" aria-expanded="false" aria-level="1" aria-owns="nav-group-functions" tabindex="-1">Functions</span>
+					<span class="DocNav-groupLabel" role="treeitem" aria-expanded="true" aria-level="1" aria-owns="nav-group-functions" tabindex="-1">Functions</span>
 					<ul role="group" id="nav-group-functions">
 						{{range .Funcs}}
 							<li role="none">
@@ -102,15 +102,15 @@ var htmlPackage = template.Must(template.New("package").Funcs(
 
 			{{if .Types}}
 				<li class="DocNav-types" role="none">
-					<span class="DocNav-groupLabel" role="treeitem" aria-expanded="false" aria-level="1" aria-owns="nav-group-types" tabindex="-1">Types</span>
+					<span class="DocNav-groupLabel" role="treeitem" aria-expanded="true" aria-level="1" aria-owns="nav-group-types" tabindex="-1">Types</span>
 					<ul role="group" id="nav-group-types">
 						{{range .Types}}
 							{{$tname := .Name}}
 							<li role="none">
 								{{if or .Funcs .Methods}}
 									{{$navgroupname := (printf "nav.group.%s" $tname)}}
 									{{$navgroupid := (safe_id $navgroupname)}}
-									<a class="DocNav-groupLabel" href="#{{$tname}}" role="treeitem" aria-expanded="false" aria-level="2" data-aria-owns="{{$navgroupid}}" tabindex="-1">type {{$tname}}</a>
+									<a class="DocNav-groupLabel" href="#{{$tname}}" role="treeitem" aria-expanded="true" aria-level="2" data-aria-owns="{{$navgroupid}}" tabindex="-1">type {{$tname}}</a>
 									<ul role="group" id="{{$navgroupid}}">
 										{{range .Funcs}}
 											<li role="none">