Add new `doc(attribute = "...")` attribute
Fixesrust-lang/rust#141123.
The implementation and purpose of this new `#[doc(attribute = "...")]` attribute is very close to `#[doc(keyword = "...")]`. Which means that luckily for us, most of the code needed was already in place and `@Noratrieb` nicely wrote a first draft that helped me implement this new attribute very fast.
Now with all this said, there is one thing I didn't do yet: adding a `rustdoc-js-std` test. I added GUI tests with search results for attributes so should be fine but I still plan on adding one for it once documentation for builtin attributes will be written into the core/std libs.
You can test it [here](https://rustdoc.crud.net/imperio/doc-attribute-attribute/foo/index.html).
cc `@Noratrieb` `@Veykril`
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