fix(layout): make content width respect page.width config (#911)

Fixes #886 - The content area in `<main>` was hardcoded to `hx:max-w-6xl`
(72rem), ignoring the `params.page.width` setting. Added a new CSS
variable `--hextra-max-content-width` that responds to the page width
config, so users can now get full-width or wide content as expected.

layouts/single.html

@@ -3,7 +3,7 @@
     {{ partial "sidebar.html" (dict "context" . "disableSidebar" true "displayPlaceholder" true) }}
     {{ partial "toc.html" . }}
     <article class="hx:w-full hx:break-words hx:flex hx:min-h-[calc(100vh-var(--navbar-height))] hx:min-w-0 hx:justify-center hx:pb-8 hx:pr-[calc(env(safe-area-inset-right)-1.5rem)]">
-      <main class="hx:w-full hx:min-w-0 hx:max-w-6xl hx:px-6 hx:pt-4 hx:md:px-12">
+      <main class="hx:w-full hx:min-w-0 hextra-max-content-width hx:px-6 hx:pt-4 hx:md:px-12">
         {{ partial "breadcrumb.html" (dict "page" . "enable" false) }}
         <br class="hx:mt-1.5 hx:text-sm" />
         {{ if .Title }}<h1 class="hx:text-center hx:mt-2 hx:text-4xl hx:font-bold hx:tracking-tight hx:text-slate-900 hx:dark:text-slate-100">{{ .Title }}</h1>{{ end }}