doc: add 'Software: Extras' page listing all third-party downloads

doc/md/packages/extras.md

@@ -0,0 +1,16 @@
+
+### Extras
+
+For convenience, some software that are not part of the official Debian distribution is included in the live system/installer. These components are listed in [Makefile.extra](https://gitlab.com/nodiscc/debian-live-config/-/blob/master/Makefile.extra):
+
+<!-- grep '# EXTRA' Makefile.extra -->
+
+ - https://github.com/scopatz/nanorc
+ - https://github.com/az0/cleanerml
+ - https://github.com/nodiscc/user.js
+ - https://github.com/EionRobb/pidgin-opensteamworks/
+ - https://github.com/yt-dlp/yt-dlp
+ - https://addons.mozilla.org/en-US/firefox/addon/keepassxc-browser/
+ - https://addons.mozilla.org/en-US/firefox/addon/cookie-autodelete/
+
+These components are not installed from APT repositories and will receive no automatic updates unless someone [packages and uploads them](https://wnpp.debian.net/) to the Debian archive. Please refer to each component's documentation for manual upgrade instructions.