Make automated pipelines run only when needed

[cncastillo]

Feature Request

Now even a change in the README trigger a Buildkite GPU run. We should be more efficient:

• Buildkite tests and benchmarks: Only when things at KomaMRIBase/, and KomaMRICore/ change (and the pipeline file). The benchmarks now depend on all of the GPU back-ends to pass which is not very helpful if they are not passing (AMDGPU and oneAPI).
• GitHub CI tests: We could generate a CI badge for each package (Separate CI for more comprehensive results #276).
  • Changes in KomaMRIBase/ should trigger KomaMRI, KomaMRICore, KomaMRIPlots and KomaMRIFiles tests
  • Changes in KomaMRICore/ should trigger KomaMRI, KomaMRICore tests
  • Changes in KomaMRIFiles/ should trigger KomaMRIFiles tests
  • Changes in KomaMRIPlots/ should trigger KomaMRI, KomaMRIPlots tests
  • Changes in src/ should trigger KomaMRI tests
• Documentation: Only changes in docs/ and examples/ or documentation tag in the PR.
• Also, Work In Progress PRs shouldn't run any pipeline.