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

1 files changed, 4 insertions, 11 deletions

diff --git a/docs/content/en/variables/page.md b/docs/content/en/variables/page.md
index 9dcbdcc43..c4ddc8200 100644
--- a/docs/content/en/variables/page.md
+++ b/docs/content/en/variables/page.md
@@ -79,8 +79,7 @@ See [`.Scratch`](/functions/scratch/) for page-scoped, writable variables.
 : the page's *kind*. Possible return values are `page`, `home`, `section`, `taxonomy`, or `taxonomyTerm`. Note that there are also `RSS`, `sitemap`, `robotsTXT`, and `404` kinds, but these are only available during the rendering of each of these respective page's kind and therefore *not* available in any of the `Pages` collections.
 
 .Language
-: a language object that points to the language's definition in the site
-`config`.
+: a language object that points to the language's definition in the site `config`. `.Language.Lang` gives you the language code.
 
 .Lastmod
 : the date the content was last modified. `.Lastmod` pulls from the `lastmod` field in a content's front matter.
@@ -93,10 +92,7 @@ See also `.ExpiryDate`, `.Date`, `.PublishDate`, and [`.GitInfo`][gitinfo].
 .LinkTitle
 : access when creating links to the content. If set, Hugo will use the `linktitle` from the front matter before `title`.
 
-.Next (deprecated)
-: In older Hugo versions this pointer went the wrong direction. Please use `.PrevPage` instead.
-
-.NextPage
+.Next
 : Pointer to the next [regular page](/variables/site/#site-pages) (sorted by Hugo's [default sort](/templates/lists#default-weight-date-linktitle-filepath)). Example: `{{if .NextPage}}{{.NextPage.Permalink}}{{end}}`.
 
 .NextInSection
@@ -119,9 +115,6 @@ See also `.ExpiryDate`, `.Date`, `.PublishDate`, and [`.GitInfo`][gitinfo].
 : the Page content stripped of HTML as a `[]string` using Go's [`strings.Fields`](https://golang.org/pkg/strings/#Fields) to split `.Plain` into a slice.
 
 .Prev (deprecated)
-: In older Hugo versions this pointer went the wrong direction. Please use `.NextPage` instead.
-
-.PrevPage
 : Pointer to the previous [regular page](/variables/site/#site-pages) (sorted by Hugo's [default sort](/templates/lists#default-weight-date-linktitle-filepath)). Example: `{{if .PrevPage}}{{.PrevPage.Permalink}}{{end}}`.
 
 .PrevInSection
@@ -130,8 +123,8 @@ See also `.ExpiryDate`, `.Date`, `.PublishDate`, and [`.GitInfo`][gitinfo].
 .PublishDate
 : the date on which the content was or will be published; `.Publishdate` pulls from the `publishdate` field in a content's front matter. See also `.ExpiryDate`, `.Date`, and `.Lastmod`.
 
-.RSSLink
-: link to the taxonomies' RSS link.
+.RSSLink (deprecated)
+: link to the page's RSS feed. This is deprecated. You should instead do something like this: `{{ with .OutputFormats.Get "RSS" }}{{ . RelPermalink }}{{ end }}`.
 
 .RawContent
 : raw markdown content without the front matter. Useful with [remarkjs.com](