docs: use mathjax library from debian package instead of CDN

By default, sphinx embeds the cloudflare CDN version of mathjax. This
is bad for privacy, webistes enforcing cross-site origin protection
and in environments with no WAN access.

Luckily there's a Debian package we can use instead.

The config is the default sphinx config used.

Reported-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

debian/control

@@ -141,6 +141,7 @@ Package: proxmox-backup-docs
 Build-Profiles: <!nodoc>
 Section: doc
 Depends: libjs-extjs,
+         libjs-mathjax,
          ${misc:Depends},
 Architecture: all
 Description: Proxmox Backup Documentation

debian/control.in

@@ -38,6 +38,7 @@ Package: proxmox-backup-docs
 Build-Profiles: <!nodoc>
 Section: doc
 Depends: libjs-extjs,
+         libjs-mathjax,
          ${misc:Depends},
 Architecture: all
 Description: Proxmox Backup Documentation

debian/proxmox-backup-docs.links

@@ -2,3 +2,4 @@
 /usr/share/javascript/extjs /usr/share/doc/proxmox-backup/html/prune-simulator/extjs
 /usr/share/javascript/extjs /usr/share/doc/proxmox-backup/html/lto-barcode/extjs
 /usr/share/javascript/extjs /usr/share/doc/proxmox-backup/html/api-viewer/extjs
+/usr/share/javascript/mathjax /usr/share/doc/proxmox-backup/html/_static/mathjax