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)

.gitlab-ci.yml

@@ -11,14 +11,14 @@ workflow:
 
 format:
   script:
-    - rustup show
+    - rustup component add rustfmt
     - cargo fmt --all --check
 
 test_stable:
   script:
-    - cargo +stable check --examples --all
-    - cargo +stable check --examples --all-features --all
-    - cargo +stable test --release
+    - cargo check --examples --all
+    - cargo check --examples --all-features --all
+    - cargo test --release
 
 test_nightly:
   script:
@@ -29,8 +29,8 @@ test_nightly:
 
 check_no_std:
   script:
-    - cargo +stable build --no-default-features --target aarch64-unknown-none
+    - cargo build --no-default-features --target aarch64-unknown-none
 
 build_no_std:
   script:
-    - cargo +stable check --examples --no-default-features --target aarch64-unknown-none
+    - cargo check --examples --no-default-features --target aarch64-unknown-none