Please remove navigation2 information from robot_devkit_docs: it is outdated and causing misinformation

[SteveMacenski]

The pages here (https://intel.github.io/robot_devkit_doc/pages/navigation.html) appear in the top of google searches for navigation2, but they are woefully out of date.

Navigation2 has its own documentation now (https://ros-planning.github.io/navigation2/) and it covers the current state of the stack. The warnings listed on your page are no longer true and will cause misinformation to new users. I've had a few people already contact me confused due to this.