Make Page an interface

The main motivation of this commit is to add a `page.Page` interface to replace the very file-oriented `hugolib.Page` struct.
This is all a preparation step for issue  #5074, "pages from other data sources".

But this also fixes a set of annoying limitations, especially related to custom output formats, and shortcodes.

Most notable changes:

* The inner content of shortcodes using the `{{%` as the outer-most delimiter will now be sent to the content renderer, e.g. Blackfriday.
  This means that any markdown will partake in the global ToC and footnote context etc.
* The Custom Output formats are now "fully virtualized". This removes many of the current limitations.
* The taxonomy list type now has a reference to the `Page` object.
  This improves the taxonomy template `.Title` situation and make common template constructs much simpler.

See #5074
Fixes #5763
Fixes #5758
Fixes #5090
Fixes #5204
Fixes #4695
Fixes #5607
Fixes #5707
Fixes #5719
Fixes #3113
Fixes #5706
Fixes #5767
Fixes #5723
Fixes #5769
Fixes #5770
Fixes #5771
Fixes #5759
Fixes #5776
Fixes #5777
Fixes #5778

tpl/tplimpl/embedded/templates/_default/rss.xml

@@ -1,3 +1,9 @@
+{{- $pages := .Data.Pages -}}
+{{- $limit := .Site.Config.Services.RSS.Limit -}}
+{{- if ge $limit 1 -}}
+{{- $pages = $pages | first $limit -}}
+{{- end -}}
+{{- printf "<?xml version=\"1.0\" encoding=\"utf-8\" standalone=\"yes\" ?>" | safeHTML }}
 <rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">
   <channel>
     <title>{{ if eq  .Title  .Site.Title }}{{ .Site.Title }}{{ else }}{{ with .Title }}{{.}} on {{ end }}{{ .Site.Title }}{{ end }}</title>
@@ -12,7 +18,7 @@
     {{ with .OutputFormats.Get "RSS" }}
 	{{ printf "<atom:link href=%q rel=\"self\" type=%q />" .Permalink .MediaType | safeHTML }}
     {{ end }}
-    {{ range .Data.Pages }}
+    {{ range $pages }}
     <item>
       <title>{{ .Title }}</title>
       <link>{{ .Permalink }}</link>

tpl/tplimpl/embedded/templates/_default/sitemap.xml

@@ -1,3 +1,4 @@
+{{ printf "<?xml version=\"1.0\" encoding=\"utf-8\" standalone=\"yes\" ?>" | safeHTML }}
 <urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
   xmlns:xhtml="http://www.w3.org/1999/xhtml">
   {{ range .Data.Pages }}
@@ -8,12 +9,12 @@
     <priority>{{ .Sitemap.Priority }}</priority>{{ end }}{{ if .IsTranslated }}{{ range .Translations }}
     <xhtml:link
                 rel="alternate"
-                hreflang="{{ .Lang }}"
+                hreflang="{{ .Language.Lang }}"
                 href="{{ .Permalink }}"
                 />{{ end }}
     <xhtml:link
                 rel="alternate"
-                hreflang="{{ .Lang }}"
+                hreflang="{{ .Language.Lang }}"
                 href="{{ .Permalink }}"
                 />{{ end }}
   </url>

tpl/tplimpl/embedded/templates/_default/sitemapindex.xml

@@ -1,3 +1,4 @@
+{{ printf "<?xml version=\"1.0\" encoding=\"utf-8\" standalone=\"yes\" ?>" | safeHTML }}
 <sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
 	{{ range . }}
 	<sitemap>