Docs: document Jupyter Gallery configuration (config placement + exhibit tile icons)

[viniciusdc]

Context

We keep re-deriving Jupyter Gallery (jupyterlab-gallery) configuration from scratch, and there's nothing in the ds-pack docs about it. Two separate threads have now hit the same wall, so this issue gathers both and asks for a proper docs page.

1. Where the config has to live - established in Cannot add contents to Jupyter Gallery #134. Setting c.GalleryManager.exhibits under jupyterhub.hub.extraConfig silently no-ops: GalleryManager only exists in the singleuser server, so hub-side config is never read. It has to be a jupyter_gallery_config.py on the singleuser jupyter config path, which in a z2jh chart means injecting it via singleuser.extraFiles.

2. Broken exhibit tile images - raised internally. A tile (e.g. "JATIC Checkmaite Tutorial") renders the browser's broken-image glyph when its icon is a remote URL that times out on a locked-down deployment. The tile image is driven by the exhibit's "icon" key; when it points at a remote asset (e.g. a GitHub raw URL) the client fetch can fail, and there's no local fallback.

What to document

A Gallery page under docs/src/content/docs/ (plus a row in the configuration.md table), covering:

• Config placement. Exhibits go in jupyter_gallery_config.py on the singleuser config path (one of jupyter --paths -> config), not hub.extraConfig. Include the verified z2jh snippet:

  singleuser:
    extraFiles:
      gallery-config:
        mountPath: /etc/jupyter/jupyter_gallery_config.py
        stringData: |
          c.GalleryManager.title = "Tutorials"
          c.GalleryManager.destination = "tutorials"
          c.GalleryManager.exhibits = [
              {"title": "Xarray", "git": "https://github.com/xarray-contrib/xarray-tutorial.git", "homepage": "https://github.com/xarray-contrib/xarray-tutorial"},
          ]

• Exhibit schema. git, title, homepage, description, icon, branch, depth, and account/token for private repos (with the PAT-via-env-var note from the upstream README).

• Tile icons / the broken-image trap. The "icon" key controls the tile image. Remote icons are fetched client-side and can time out on restricted-egress deployments, leaving a broken-image placeholder with no fallback. Recommend pointing icon at a locally-served asset rather than a remote URL on such deployments.

Related / forward-looking

• From Cannot add contents to Jupyter Gallery #134: a first-class gallery.exhibits values key would remove the hand-rolled singleuser config file entirely. If that lands, the docs collapse to the values key - worth noting so the page doesn't ossify around the workaround.
• Upstream config reference: jupyterlab-gallery README GalleryManager traitlets.

[viniciusdc]

One thing worth calling out explicitly in the docs: for air-gapped or restricted-egress deployments, an exhibit whose "icon" points at a remote URL (e.g. a GitHub raw link) will fail to load, since the client can't reach the outside network - you get the broken-image placeholder with no fallback.

The recommendation for those environments is to define a local icon instead - point "icon" at an asset served from within the deployment (bundled into the singleuser image, or served from the same origin) so the tile renders without any external fetch. Worth documenting alongside the exhibit schema so deployers on locked-down clusters don't hit this by default.