feat: add options for page specific styling or integrations

Page's ID applied to body as `data-wc-page-id`. Can be accessed via CSS and JS to perform page-specific styling or integrations.

Also, introduces new page parameters, similar to widget params:
design.css_style
design.css_class

Close #1975

wowchemy/layouts/_default/baseof.html

@@ -6,7 +6,7 @@
 
 {{ $show_navbar := site.Params.main_menu.enable | default true }}
 {{- $highlight_active_link := site.Params.main_menu.highlight_active_link | default true -}}
-<body id="top" data-spy="scroll" {{ if $show_navbar }}data-offset="70"{{end}} data-target="{{ if or .IsHome (eq .Type "widget_page") | and $highlight_active_link }}#navbar-main{{else}}#TableOfContents{{end}}" class="page-wrapper {{ if not (.Scratch.Get "light") }}dark{{end}} {{ if not $show_navbar }}no-navbar{{end}}">
+<body id="top" data-spy="scroll" {{ if $show_navbar }}data-offset="70"{{end}} data-target="{{ if or .IsHome (eq .Type "widget_page") | and $highlight_active_link }}#navbar-main{{else}}#TableOfContents{{end}}" class="page-wrapper {{with .Params.design.css_class}}{{.}}{{end}} {{ if not (.Scratch.Get "light") }}dark{{end}} {{ if not $show_navbar }}no-navbar{{end}}" {{with .File}}data-wc-page-id="{{.File.UniqueID}}"{{end}} {{with .Params.design.css_style}}style="{{. | safeCSS}}"{{end}}>
 
   {{/* Initialise Wowchemy. */}}
   {{ $js_license := printf "/*! Wowchemy v%s | https://wowchemy.com/ */\n" site.Data.wowchemy.version }}