ci: enhance CI workflows and make them consistent (tox and GH actions)

[egparedes]

• Simplify conditional specification of commands in tox configurations (based on the trick mentioned in tox4: Factor conditions are not considered for commands tox-dev/tox#2002 (comment)) and homogenize factor matrices across different github workflows.
• Add pytest-custom-exit-code plugin to ignore errors when no tests are collected in a pytest run (useful when using filters). Check pytest documentation about exit codes (https://docs.pytest.org/en/stable/reference/exit-codes.html) for more details.
• Update outdated links in the CI development documentation.

[DropD]

Looks simpler and more readable. I think it can be merged as soon as the CI has passed.

[egparedes]

cscs-ci run default