This commit unifies the core internal page index for all page kinds.
This enables the `ref` and `relref` shortcodes to support all pages kinds, and adds a new page-relative `.GetPage` method with simplified signature.
See #4147
See #4727
See #4728
See #4728
See #4726
See #4652
As an example:
```html
{{ $pages := .Site.RegularPages | lang.Merge $frSite.RegularPages | lang.Merge $enSite.RegularPages }}
```
Will "fill in the gaps" in the current site with, from left to right, content from the French site, and lastly the English.
Fixes#4463