Rollup merge of rust-lang#92602 - jsha:source-link-2, r=GuillaumeGomez

Make source links look cleaner

Change from syntaxy-looking [src] to the plain word "source".

Change the syntaxy-looking `[-]` at the top of the page to say "collapse".

Reduce opacity of rightside content.

Part of rust-lang#59851

r? `@GuillaumeGomez`

Demo: https://rustdoc.crud.net/jsha/source-link-2/std/string/struct.String.html

[Discussed on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/266220-rustdoc/topic/display.20of.20source.20link).

Author: matthiaskrgr

std/src/lib.rs

@@ -35,8 +35,8 @@
 //! development you may want to press the `[-]` button near the top of the
 //! page to collapse it into a more skimmable view.
 //!
-//! While you are looking at that `[-]` button also notice the `[src]`
-//! button. Rust's API documentation comes with the source code and you are
+//! While you are looking at that `[-]` button also notice the `source`
+//! link. Rust's API documentation comes with the source code and you are
 //! encouraged to read it. The standard library source is generally high
 //! quality and a peek behind the curtains is often enlightening.
 //!