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.:
```
{{ $related := site.RegularPages.Related .Page }}
```
But, and this is important, you need to include the shortcode using the `{{<` delimiter. Not doing so will create infinite loops and timeouts.
This commit also:
* Adds two new methods to Page: Fragments (can also be used to build ToC) and HeadingsFiltered (this is only used in Related Content with
index type `fragments` and `enableFilter` set to true.
* Consolidates all `.Related*` methods into one, which takes either a `Page` or an options map as its only argument.
* Add `context.Context` to all of the content related Page API. Turns out it wasn't strictly needed for this particular feature, but it will
soon become usefil, e.g. in #9339.
Closes #10711
Updates #9339
Updates #10725
This commit pulls most of the image related logic into its own package, to make it easier to reason about and extend.
This is also a rewrite of the transformation logic used in Hugo Pipes, mostly to allow constructs like the one below:
{{ ($myimg | fingerprint ).Width }}
Fixes #5903
Fixes #6234
Fixes #6266
