Implement dynamic photo gallery with slider and modular structure

[a-latyshev]

This pull request introduces a new dynamic photo gallery feature for the conference website, specifically for the FEniCS 2026 page. The gallery is automatically populated by fetching images from a remote URL and updating a local JSON file, which is then used to render a modern, interactive slider and lightbox experience. The implementation includes a Python utility for updating image lists, new HTML and JavaScript for the gallery, and supporting CSS for styling.

New Dynamic Photo Gallery Feature

• Added a gallery_url and gallery_data field to the conference/2026.md frontmatter to specify the remote image source and local JSON data file for the gallery. The gallery is rendered using a new {% include gallery.html %} section on the page. [1] [2]

• Created _includes/gallery.html with JavaScript to dynamically fetch image URLs from the specified JSON file, render a scrollable thumbnail gallery, and provide a lightbox modal with navigation and keyboard support.

• Added custom styles in assets/css/customsty.css for the gallery slider, thumbnail grid, slider buttons, and lightbox modal, including responsive design tweaks.

Automation and Workflow

• Introduced _tools/update_galleries.py, a Python script that crawls markdown files for gallery_url and gallery_data, fetches image links from the specified URL, and writes them to the corresponding JSON file. This script also cleans up old gallery_images entries in the frontmatter.

• Updated the GitHub Actions workflow (.github/workflows/build.yml) to run the gallery update script automatically before building the website, ensuring image galleries are always up to date.