This rewrites std_links for two major changes:

- Uses pulldown_cmark to parse the markdown instead of using regular
  expressions. The RegEx approach was just too unreliable.
- Call rustdoc only once, instead of once per chapter. The overhead of
  calling rustdoc is significant, and calling it for each chapter was
  extremely slow.

  Unfortunately the error messages are a little worse now, since it
  doesn't show which chapter a bad link came from. I'm uncertain how to
  make that better.

This should speed things up a little, particularly when working locally.

This removes the explicit links to the standard library. In particular,
this makes it nicer to view locally since you can set SPEC_RELATIVE=0 to
make the links work.

There are a bunch of changes to the actual URL because rustdoc resolves
re-exports to link to the original definition instead of the re-export
site. From what I can tell, everything should otherwise be the same.

Not all links were able to be converted due to some limitations in
rustdoc, such as:

- Links to rexports from std_arch don't work due to rust-lang/rust#96506.
- Links to keywords aren't supported.
- Links to trait impls where the trait is not in the prelude doesn't work (they must be in scope).

In this commit, we improve some verbiage.

There were some errors in the formatting and the content of the
grammar for unsafe attributes.  Let's fix those.

Let's describe unsafe attributes in the chapter on unsafety.

The _Attr_ production is used for `cfg_attr` attribute and the `meta`
macro fragment specifier, and those need to accept the new `unsafe`
syntax.

This will be required in 2024.

Add some basic docs for unsafe attrs

It's better to allocate for the smaller prefix that we're extracting
than to allocate repeatedly for the tail items, especially as
`split_off` doesn't shrink the capacity of what we go on to insert.

Rewrite the automatic std link translation, and switch to automatic links