docs: promote /web from develop — v1.2.3 changelog + Backups video + install page refresh + PBS keyfile docs polish

Consolidates the /web changes that were made on develop for the v1.2.3 stable
release, so the release PR (develop → main) can focus on version.txt +
AppImage + shell scripts without web conflicts.

Contents:

- CHANGELOG.md + web/data/changelog/es.md — v1.2.3 stable entry prepended
  (bullets + tables, not paragraphs) with the release header image.
- images/ProxMenux_backup.png — new v1.2.3 release header image.
- web/app/[locale]/docs/backup-restore/page.tsx +
  web/messages/{en,es}/docs/backup-restore/index.json — new Video walkthrough
  block for the YouTube demo of the Backups feature (videoId K7A1KtOe4IQ),
  embedded high on the overview page via the shared YouTubeEmbed component.
- web/app/[locale]/docs/installation/page.tsx +
  web/messages/{en,es}/docs/installation.json +
  web/public/install/install.png — stale Translation-flavour and Python-from-
  PyPI mentions removed from the install page; screenshot refreshed and moved
  to local /install/install.png (was pointing at the legacy GitHub Pages URL).
- web/messages/es/docs/backup-restore/destinations/pbs.json — minor polish
  aligned with PR #247 already on main.
- web/public/images/docs/backup-restore/pbs-paired-backup-groups.png — updated
  screenshot to match the reworked PBS-paired-group naming shipped in the beta
  cycle.
- images/logos_txt/jc_channel.txt — new asset for the contributors page.

web/messages/en/docs/backup-restore/index.json

@@ -16,6 +16,12 @@
     "title": "What it is, in one paragraph",
     "body": "A ProxMenux backup captures the complete state of a Proxmox host: <strong>the filesystem</strong> (relevant directories under <code>/etc</code>, <code>/root</code>, <code>/var/lib/pve-cluster</code>, plus optional custom paths), <strong>a structured manifest</strong> (JSON with the detected hardware, kernel parameters, network layout, ZFS state, users and cron entries), and <strong>an application inventory</strong> (all packages marked as manually installed by APT, plus the list of components installed by ProxMenux with their exact versions). Any of the three supported destinations — local archive, Proxmox Backup Server (recommended) or Borg — receives the same self-contained payload, and any of them can hydrate the target host without depending on the source being reachable at restore time."
   },
+  "video": {
+    "heading": "Video walkthrough",
+    "intro": "Prefer watching Backup & Restore in action before configuring it? This walkthrough covers a full end-to-end demo of the feature in ProxMenux.",
+    "title": "ProxMenux — Backup & Restore walkthrough",
+    "caption": "External video on YouTube. Plays in privacy-enhanced mode (no cookies until you press play)."
+  },
   "whatItIsNot": {
     "heading": "What it is not",
     "intro": "The section covers <strong>host-level</strong> backup and restore: the Proxmox installation itself, not the workloads running on top of it.",