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)

added 1 commit

• a7dc4389 - DEBUG: show the rustup setup

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added 3 commits

• a285ac85 - revert the debug
• 1e65baee - install `rustfmt` in the default toolchain of `format`
• 56dee870 - use the default stable toolchain

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added 1 commit

• 792ba9cb - format the code with `cargo fmt --all`

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changed the description

added 1 commit

• 088c382a - do not deny the missing docs

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added 2 commits

• 7c9912c3 - Revert "do not deny the missing docs"
• 54a094e7 - add the problematic missing docs as TODOs

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changed the description

changed the description

added 1 commit

• 3c526318 - fix the naming confusion in the last `*_no_std` tests

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added 1 commit

• 4976c462 - comment out the failing `build_no_std` job

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changed the description

enabled an automatic merge when the pipeline for 4976c462 succeeds

merged