rustdoc: add support for macro_rules macros of multiple kinds

[notriddle]

This adds support for advanced RFC 3697 and 3698 macros, that look like this:

/// Doc Comment.
macro_rules! NAME {
    attr(key = $value:literal) ($attached:item) => { ... };
    derive() ($attached:item) => { ... };
    ($bang:tt) => { ... };
}

And can be used like this:

/*attr*/ #[NAME]
/*derive*/ #[derive(NAME)]
/*bang*/ NAME!{}

The basic problem is that there are three different ways to use this macro, and, historically, these three ways were implemented by fully-separated proc macro items with separate doc comments. We want these new declarative macros to work the same way, so they appear in the same section of the module table of contents and can be filtered for in the search engine.

This commit makes the feature work by generating separate pages for each macro kind. These separate pages appear in separate sections of the module table of contents, produce separate search results, and have duplicate copies of the doc comment.

The following features would also be helpful, but are left for future commits to implement:

• To improve the produced documentation, we should probably add support for adding /// attributes to macro rules arms, so that the different syntaxes can actually be documented separately.

• This commit doesn't test intra-doc links.

Parts of #145458 are directly used in this code.

cc @joshtriplett @GuillaumeGomez @lolbinarycat

[GuillaumeGomez]

Although I find the result of this approach less good (ie, duplicated files generated for the same macro), considering how much easier the implementation is, I'd prefer for this one to be merged. I'll close #145458.

[GuillaumeGomez]

Btw, the PR looks ready to me so feel free to r=me unless you want to make more changes.