Add additional CI checks

[prokopyl]

This PR adds a variety of checks to the CI:

• Check that all code and examples build without warnings using -D warnings. This is done for all platforms, with or without the OpenGL feature.
• Check that all documentation is valid, and that no documentation links are broken, using cargo doc with -D warnings.
• Run clippy with -D warnings (but otherwise only the default lints).
• Check formatting using rustfmt.

This PR also applies some light formatting and code fixes that were detected by rustfmt and clippy.

This also makes some updates to the actions used:

• Update actions/checkout to v4
• Replace actions-rs/toolchain (which is not maintained anymore) to dtolnay/rust-toolchain instead

Some further work that is not covered by this PR:

• The missing-safety-doc lint has been temporarily disabled. This will be re-enabled in another PR that will add the proper safety docs to the affected functions.
• cargo-deny has been discussed, but I prefer adding it in a later PR as it can be its own can of worms regarding licenses and dependencies.