static/frontend: ensure page content fits across >= 320 CSS pixels

Ensure main page content is readable at 320 CSS pixels without the need
for horizontal scrolling. This is important for documentation users that
may require zoom functionality in order to read content. WCAG 2.2
defines 320 pixels as the minimum width reasonably achieveable by site
authors.

See https://www.w3.org/WAI/WCAG22/Understanding/reflow.html.

Fixes #66996.

Change-Id: Ic3d02bffac23798bcd86d28560845fa91ce831db
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/581236
Reviewed-by: Jonathan Amsterdam <jba@google.com>
kokoro-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jamal Carvalho <jamalcarvalho@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Findley <rfindley@google.com>

static/frontend/unit/main/_meta.css

@@ -7,7 +7,6 @@
 .UnitMeta {
   display: grid;
   gap: 1rem 2rem;
-  grid-template-columns: max-content auto;
   white-space: nowrap;
 }
 

static/shared/shared.css

@@ -73,7 +73,7 @@ body {
   display: flex;
   flex-direction: column;
   min-height: 100vh;
-  min-width: 23.5rem;
+  min-width: 20rem;
 
   /**
    * This is used to programatically detect whether overflow needs to be altered