Update installation instructions for source build (backport #967)

[mergify]

I think there is some information missing, if someone wants to build the driver from source on a non-rolling ROS distro.

Relates to https://robotics.stackexchange.com/questions/110644/ros2-ur-driver-package-failed-ur-controllers/110654#110654

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This is an automatic backport of pull request #967 done by Mergify.

[mergify]

Cherry-pick of 2d5e315 has failed:

On branch mergify/bp/humble/pr-967
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit 2d5e315.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   ur_robot_driver/doc/installation/installation.rst

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[fmauch]

Seems like for humble we never restructured things that much, see #884