[llvm] [llvm][docs] Expand HowToAddABuilder with guidance on testing locally (PR #115024)

[Alex Bradbury via llvm-commits]

asb wrote:

Thank you, made those final tweaks and reflected the change to BUILDBOT_TEST now that change in llvm-zorg landed.

I'll hold off merging until https://github.com/llvm/llvm-zorg/pull/293 as the instructions don't work without that fix sadly.

https://github.com/llvm/llvm-project/pull/115024