Add support for opening docs.rs for symbol under cursor

[liamwh]

Feature description

Like so: https://www.reddit.com/r/rust/comments/w7cf7d/rustanalyzer_tip_open_docsrs_for_the_symbol_under/

I'm new to NeoVim, please forgive me if there is already an easy way to do this.

[mrcjkb]

The feature was already there, but I had forgotten to add the command 😅