Filter out `package.documentation` links to docs.rs itself

[Nemo157]

On e.g. https://docs.rs/crate/diesel/latest there's a "Documentation" link in the left-hand bar that just points to https://docs.rs/diesel. This is mainly shown for cases where a crate has external documentation, e.g. https://docs.rs/crate/diesel/0.13.0 which links to http://docs.diesel.rs/, so we should suppress showing it for cases that just link back to docs.rs.

[syphar]

mentoring instructions:

• when loading the crate details from the database (in CrateDetails::new in web::crate_details) we also load documentation_url
• here we want to ignore the documentation URL when it starts with https://docs.rs, so CrateDetails.documentation_url is None in this case
• add a test for this, just on CrateDetails is fine

the interfaces already show the link only when there is an URL.