fix: generate responsive hero background images (#970)

Fixes #362 by generating responsive hero background image CSS when the
featured image is available as a Hugo Page Resource.

* Adds `layouts/_partials/func/GetFeaturedImageResource.html` to resolve
the same featured-image candidates as a Hugo image resource.
* Updates `layouts/_partials/page-header.html` to generate resized hero
background images with media queries.
* Adds the optional `[params.ananke] responsive_image_widths` setting,
defaulting to `[480, 960, 1440, 1920]`.
* Keeps the existing behaviour for static paths, remote paths, SVG
files, and images Hugo cannot resize.

Documentation PR: gohugo-ananke/documentation#14

* Checked issue #362 and its comments, including the earlier unmerged PR
* Checked `CONTRIBUTING.md` and used the required `development` base
branch.
* Compared `issues/362` against `development`; the branch is cleanly
ahead by two commits and behind by zero commits.
* Unable to run a local Hugo build in this environment because direct
`git clone` failed with DNS resolution for `github.com`; verified
changes by repository API inspection and compare output instead.

This implementation is intentionally narrower than the older closed PR
and avoids changing summary images or the figure shortcode.