In Hugo #8776 we added `pageRef`, a way to connect menu items in site config to pages.
This means that you now can have both a Page and a configured URL.
Having the configured URL as a fallback if the Page isn't found is obviously more useful, especially in multilingual sites.
See #8776
.Site.Menus is mutated when it is sorted for now and this causes concurrency problem (#7594)
In this patch, each related sort function copies Menu before sorting to prevent
race condition.
Pages already have such a sort and cache logic and this patch is identical to it.
Closes#7594
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
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 #5074Fixes#5763Fixes#5758Fixes#5090Fixes#5204Fixes#4695Fixes#5607Fixes#5707Fixes#5719Fixes#3113Fixes#5706Fixes#5767Fixes#5723Fixes#5769Fixes#5770Fixes#5771Fixes#5759Fixes#5776Fixes#5777Fixes#5778
