Remove `serde` and `webdriver` from `default` features

[madsmtm]

I suspect most users will not want these enabled, but it's very easy to accidentally do so when depending on keyboard-types. Not that it's a big problem, because both deps are small and trusted, but it's still a bit annoying.

I've also enabled doc_auto_cfg to ensure that the docs on docs.rs make it clear that feature flags are required to enable these.