Broken links in Nixpkgs unstable Manual

[aktaboot]

Problem

Found broken links in the Nixpkgs unstable manual: here's the list

• https://mike-fabian.github.io/ibus-typing-booster/documentation.html
• https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/networking/mailreaders/thunderbird/default.nix
• https://developer-docs.citrix.com/projects/receiver-for-linux-command-reference/en/13.7/
• https://developer.gnome.org/gdk-pixbuf/stable/
• https://www.citrix.de/downloads/workspace-app/linux/workspace-app-for-linux-latest.html
• https://elementary.io/docs/human-interface-guidelines
• https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/beam-modules/elixir_ls.nix

#208868 addresses one

Checklist

• checked latest Nixpkgs manual (source) and latest NixOS manual (source)
• checked open documentation issues for possible duplicates
• checked open documentation pull requests for possible solutions

[ncfavier]

#212418