The main topic of this commit is that you can now index fragments (content heading identifiers) when calling `.Related`.
You can do this by:
* Configure one or more indices with type `fragments`
* The name of those index configurations maps to an (optional) front matter slice with fragment references. This allows you to link
page<->fragment and page<->page.
* This also will index all the fragments (heading identifiers) of the pages.
It's also possible to use type `fragments` indices in shortcode, e.g.:
```
{{ $related := site.RegularPages.Related .Page }}
```
But, and this is important, you need to include the shortcode using the `{{<` delimiter. Not doing so will create infinite loops and timeouts.
This commit also:
* Adds two new methods to Page: Fragments (can also be used to build ToC) and HeadingsFiltered (this is only used in Related Content with
index type `fragments` and `enableFilter` set to true.
* Consolidates all `.Related*` methods into one, which takes either a `Page` or an options map as its only argument.
* Add `context.Context` to all of the content related Page API. Turns out it wasn't strictly needed for this particular feature, but it will
soon become usefil, e.g. in #9339.
Closes#10711
Updates #9339
Updates #10725
This commit also
* revises the change detection for templates used by content files in server mode.
* Adds a Page.RenderString method
Fixes#6545Fixes#4663Closes#6043
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
