In Hugo `0.55` we introduced the `permalinkable` config attribute on Output Format, default enabled for `AMP` and `HTML`.
This meant that a Page could have different `RelPermalink` and `Permalink` depending on the rendering format.
The menu `URL` did not reflect that fact.
Fixes #5849
This stopped working in Hugo 0.55:
```bash
---
menu: "main"
---
```
This was also the case for using a slice of menu entries.
This still worked:
---
menu:
main:
weight: 30
---
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
