Another round of docs fix + cleanups

[slinkydeveloper]

• This includes several docs additions and fixes, following Check the checklist of rust library design api #79
• This makes the *Ext traits in integration crates sealed, as explained in https://rust-lang.github.io/api-guidelines/future-proofing.html#c-sealed
• Renamed cloudevents_sdk_actix_web::RequestExt to cloudevents_sdk_actix_web::HttpRequestExt
• Made some methods of Attributes pub(crate) (although the whole data structured should become at some point pub(crate), it's not even buildable outside this crate)
• Removed Data::from_* methods (they are not used and confusing)
• Fixed all samples (some of them were not compiling)
• Added https://rust-lang.github.io/api-guidelines/documentation.html#c-html-root, this should fix Some docs intra links seems broken #77