Update getting_started.rst

[timassman]

• Added instruction to checkout humble branch when using ROS humble
• Added instruction to remove previously installed moveit binaries

Description

While following the tutorials myself, after waiting for the build, RViz crashed, unclear why. It turned out there is a specific "humble" branch. Also I had previously installed moveit binaries. There were conflicts. It took me quite some time, especially because the build takes so much time and the errors are unclear.

I propose to add these instructions to prevent other users from running into the same issues.

Checklist

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