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

resources/resource_test.go

@@ -16,7 +16,6 @@ package resources
 import (
 	"fmt"
 	"math/rand"
-	"path"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -45,9 +44,8 @@ func TestGenericResourceWithLinkFacory(t *testing.T) {
 	assert := require.New(t)
 	spec := newTestResourceSpec(assert)
 
-	factory := func(s string) string {
-		return path.Join("/foo", s)
-	}
+	factory := newTargetPaths("/foo")
+
 	r := spec.newGenericResource(nil, factory, nil, "/a/foo.css", "foo.css", media.CSSType)
 
 	assert.Equal("https://example.com/foo/foo.css", r.Permalink())