In page.NewOutputFormat, we take an output.Format f and use it to
create a page.OutputFormat. If the format is canonical, we assign
the final OutputFormat's Rel to "canonical" rather than using
f.Rel. However, this leads to unexpected behavior for custom
output formats, where a user can define a "rel" for a format
via the config file.
For example, the standard for "humans.txt" files requires using
rel="author" in HTML "link" elements. Meanwhile, humans.txt is
usually the only format used for its content. As a result, for
Hugo configurations that define a humans.txt custom output format,
Hugo will render "link" elements to content in this format with
rel="canonical," rather than "author" as required by the standard.
This commit changes page.NewOutputFormat to check whether a given
format is user defined and, if so, skips assigning Rel to
"canonical," even if isCanonical is true.
Fixes#8030
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
