Add tutorial: How to have supplemental custom rosdep keys, on top of the standard index

[emersonknapp]

There are tutorials on how to fork rosdistro in its entirety, but no information on how you can supplement the rosdistro index with a custom set of extra keys. This is possible using the sources.list.d model just like with APT and other of those types of priority-based linux lists directories.

Quick thought of contents:

1. Add new /etc/ros/rosdep/sources.list.d/10-custom.list
2. Example content for that file, how to point to URL or local filesystem file
3. What goes into the custom keys file, links and small example

Some references that lead partway there:

• https://docs.ros.org/en/rolling/How-To-Guides/Using-Custom-Rosdistro.html
• https://github.com/ros-infrastructure/ros_buildfarm/blob/master/doc/custom_rosdistro.rst
• https://docs.ros.org/en/independent/api/rosdep/html/contributing_rules.html

[ros-discourse]

This issue has been mentioned on Open Robotics Discourse. There might be relevant details there:

https://discourse.openrobotics.org/t/ros-pmc-minutes-for-august-12-2025/49548/3