CI: add same jobs as in original GitHub fork

translate the GitHub CI from the original repo to .gitlab-ci.yml

Note
see .github/workflows/ci.yml

this new CI should run the same tests as in the original fork.

Note
this MR also

• formats the whole workspace with cargo fmt --all in 792ba9cb (see jobs/8238)
• add missing doc introduced by 5d91c12b in 54a094e7 (see jobs/8244)

noticeable differences

• uses the default stable toolchain instead of being explicit
• does not run the build_no_std job (see jobs/8262)