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/transform.go

@@ -320,7 +320,7 @@ func (r *transformedResource) transform(setContent, publish bool) (err error) {
 			key = key + "_" + v.transformation.Key().key()
 		case permalinker:
 			r.linker = v
-			p := v.targetPath()
+			p := v.TargetPath()
 			if p == "" {
 				panic("target path needed for key creation")
 			}
@@ -375,7 +375,7 @@ func (r *transformedResource) transform(setContent, publish bool) (err error) {
 	tctx.To = b1
 
 	if r.linker != nil {
-		tctx.InPath = r.linker.targetPath()
+		tctx.InPath = r.linker.TargetPath()
 		tctx.SourcePath = tctx.InPath
 	}