Only run github action if related files are changed

[montyly]

For example when we change markdown files that are not related to Echidna, we should not run all the echidna tests

[ggrieco-tob]

Perhaps @elopez has some idea on how to solve this, since it looks like a "generic CI issue".

[elopez]

paths and paths-ignore can be used to implement this: https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onpushpull_requestpull_request_targetpathspaths-ignore

[tuturu-tech]

We could use something like https://github.com/dorny/paths-filter or https://github.com/MarceloPrado/has-changed-path to conditionally execute the action steps. E.g. if only Echidna exercise 4 is updated then there's no need to run all exercise tests in the Echidna action.