Add resource-tiles shortcode

Displays a page's file resources as a responsive grid of cards with a
thumbnail (explicit param, name-matched image, or a generic file icon),
title, size and download link. Meant for a downloads page offering a
handful of PDFs.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -33,6 +33,61 @@ first line of a page's content to get a hero flush against the navbar.
 Optional params: `title`, `height` (a CSS length such as `60vh`, which
 crops the image with `object-fit: cover`) and `class`.
 
+### `resource-tiles`
+
+Displays a page's file resources as a responsive grid of cards
+("tiles"), each with a thumbnail, a title, the file size and a download
+link. Meant for a downloads page that offers a handful of PDFs,
+spreadsheets, archives, …
+
+```
+{{< resource-tiles >}}
+```
+
+Params:
+
+| Param   | Default | Meaning                                                 |
+|---------|---------|--------------------------------------------------------|
+| `match` | `*.pdf` | glob selecting which page resources to show             |
+| `cols`  | `3`     | number of columns on large screens                     |
+| `sort`  | —       | resource field to order by (`title`, `name`, `weight`) |
+| `class` | —       | extra CSS classes for the wrapping row                 |
+
+Thumbnails are resolved per resource, in order of preference:
+
+1. a `thumbnail` param on the resource, pointing at an image in the same
+   page bundle;
+2. an image resource whose name matches the file, e.g. `report.png` or
+   `report.pdf.png` next to `report.pdf`;
+3. a generic file icon showing the upper-cased extension.
+
+Hugo cannot rasterise PDFs itself, so first-page previews have to be
+produced as a build step. With poppler-utils installed, generating a
+`<name>.png` next to every PDF under `content/` is a one-liner:
+
+```sh
+find content -name '*.pdf' -exec sh -c \
+  'pdftocairo -png -singlefile -scale-to-x 600 -scale-to-y -1 "$1" "${1%.pdf}"' _ {} \;
+```
+
+Run it before `hugo`; the tiles then use these as thumbnails
+automatically. (`pdftoppm` takes the same flags; `mutool draw` is an
+alternative.)
+
+Per-resource title, description and thumbnail come from the page's front
+matter:
+
+```yaml
+resources:
+  - src: report.pdf
+    title: Annual report
+    params:
+      description: Everything that happened last year.
+      thumbnail: report-cover.png
+```
+
+The whole tile is a click target (Bootstrap `.stretched-link`).
+
 
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