[WIP] CI: handle required workflows when skipped

[EZoni]

Work-in-progress PR to try out various solutions to handle skipped but required CI workflows properly, e.g., when only documentation is changed (trivial one-word change in this PR).

Follow-up to #5387. Originally tried some of these in #5386.

[EZoni]

@WeiqunZhang

With regard to #5387 (comment) as well as what was done in #5387 and AMReX-Codes/amrex#4197, I tried several approaches including (i) adding dependencies between workflows through workflow_run, (ii) using reusable workflows through workflow_call, and others, but all failed at working around GitHub's handling of required checks that are skipped.

The approach implemented here, based on a custom bash script that is called from within each workflow before running the expected checks, seems to be the only one that I got to make work and that doesn't rely on third-party code (hence maybe easier to maintain, debug, etc.).

Let me know if this seems like a good enough solution to you.

Note that for the clang tidy checks, organized with a strategy matrix approach, we need to apply the if conditions at each step of the job, rather than at the job level, otherwise each workflow defined by the matrix remains pending.

[EZoni]

Please see the checks for 488add9 or 6636191 to verify how this works when all checks are skipped.

[EZoni]

Not clear how the new skip_checks job behaves when a given workflow is triggered by a push event (e.g., when merging a PR into the main branch), rather than a pull_request event (e.g., when pushing commits to an open PR, like here)...