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
Adds a config entry to Directory Structure
Adds a Hugo.Environment entry to variables/hugo
Edit the configuration page to wrap first part under a Configuration File heading and introduce a second heading, Configuration Directory to present the feature.
While Hugo does not care if the section is named "post" or "posts",
this commit normalizes the whole Hugo documentation to use "posts"
when used as an example for a section name to prevent user confusion.
Fixes https://github.com/gohugoio/hugoDocs/issues/720.
Also highlight the use of `site.Params.mainSections`.
Introduce new page position variables in order to fix the ordering issue
of `.Next` and `.Prev` while also allowing an upgrade path via
deprecation.
`.NextInSection` becomes `.NextPageInSection`.
`.PrevInSection` becomes `.PrevPageInSection`.
`.Next` becomes a function returning `.PrevPage`.
`.Prev` becomes a function returning `.NextPage`.
Fixes#1061
