Add a rustdoc commandline parameter to disable generation of source code pages

[sdroege]

Kind of related to #67804 and the html_no_source attribute.

The behaviour would be the same as the latter but wouldn't require any source code changes. My use-case here is to generate docs on the CI and reduce the size (400MB just for docs for each version is a bit much), but if people want to generate docs locally they should still be able to get the source code pages if they want.

@GuillaumeGomez suggested to create an issue for this.

[GuillaumeGomez]

To reduce the global documentation size, I suggested in #112379 to group items that don't have impls by kinds. Would require an RFC though.

In any case, I think it's a good idea to add an option to have this done in the command line.