feat: set documentation_url to None when url starts with https://docs.rs

[zahidkizmaz]

Description

Issue: #2184

Documentation URL is mainly used for the external documentation websites. When the url starts with https://docs.rs we don't need to include it on the page.

How to test

With branch checked out run:

cargo run -- build crate diesel 0.13.0
cargo run -- build crate diesel 2.2.4

• go to http://localhost:3000/crate/diesel/0.13.0 Make sure that Links section -> Documentation points to http://docs.diesel.rs/
• go to http://localhost:3000/crate/diesel/2.2.4 Make sure that Links section doesn't have Documentation anymore.

[syphar]

only one small change needed, thanks!