De-duplicate RT kernel build instructions

[carlossvg]

Context from ros-realtime/rtwg_documentation#29:

There's a guide for building a RT kernel here: https://real-time-working-group.readthedocs.io/en/latest/Guides/Real-Time-Operating-System-Setup/Real-Time-Linux/build_rt_kernel.html

but the instructions here are slightly more up-to-date/better: https://github.com/ros-realtime/rt-kernel-docker-builder

Should the guide be updated with the text from the README? And to de-duplicate we can remove it from the README and point to the guide in the documentation.

Acceptance criteria

• Move instructions to ros-realtime.github.io/
• Remove instructions from https://github.com/ros-realtime/linux-real-time-kernel-builder readme and add new URL there