Add a documentation build step to CI

[fjarri]

Even though documentation is build automatically by docs.rs on release, it uses a non-standard environment to build docs, so it would be useful to check in a CI action that docs can be built correctly.

In particular:

• it uses the nightly Rust
• it uses a specific configuration flag (docsrs, see package.metadata.docs.rs section in Cargo.toml), which enables doc_cfg feature and corresponding doc() attributes
• it uses specific features (also set in package.metadata.docs.rs section in Cargo.toml)

For more info see https://docs.rs/about/builds