test sdist in CI
- Replace "doc" stage with "build" stage
- Add "test_build" stage
- "build" stage: build sdist and docs for main branch
- "test_build" stage: test sdist for main branch
Example of "build" and "test_build" stage: https://gitlab.esrf.fr/workflow/ewoks/ewoks/-/pipelines/52248
Edited by Wout De Nolf