https://github.com/rust-lang/rust/pull/116412 will remove support for
compiler plugins from rustc, which includes the entry in The Rust
Unstable Book. This commit removes a link to that entry so it won't be
broken when that PR merges.
Instead of placing the link to the recommended resource i.e. the second
edition in second place, push it up to the top of the list. In fact,
push the link to the first edition always at the bottom.
To emphasise even more that's where new visitors should go, bolden the
first link, and make the first-ed link smaller.
All this as well as a slight edit and simplification of the copy is
designed to make the new and recommended resources the primary path
readers take, without hiding the old resource completely.
To improve immediate value and reduce discard-because-not-useful factor
upon arriving at a page which does not contain the content you were
expecting, add a small blurb taken/adapted from the newer source,
optionally accompanied by a code sample describing the feature.
However, due to the transitive nature of this resource, there must be no
content here that isn't available elsewhere.
And reword the blurb to include what was previously in the title.
This makes it easier to see that one has arrived to the right place:
beforehand, the only indication was through the link target for the
first edition; it was nowhere in the copy itself for most pages.
