Move attribute and keyword docs from std to core

[jschillem]

Move the documentation for attributes and keywords into the core crate. Apart from strictly moving the module, I had to make a few small changes to certain docs to avoid using std types when possible, as well as fixing a few suggestions related to linking to primitives.

Pre-requisite for work on #157604.

r? @GuillaumeGomez

Verified documentation using:
./x doc library/core and ./x doc library/std, as well as running doc-tests for both crates.

[rustbot]

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[GuillaumeGomez]

Thanks!

@bors r+ rollup

[rust-bors]

📌 Commit e5d79ed has been approved by GuillaumeGomez

It is now in the queue for this repository.