Auto merge of #9541 - Alexendoo:declare-proc-macro, r=flip1995

Generate lint categories and explanations with `declare_clippy_lint`

This means contributors will no longer have to run `cargo dev update_lints` after changing a lints documentation or its category, which may also mean fewer merge conflicts in general

It works by swapping `declare_clippy_lint` out for a `proc_macro` of the same name. The proc macro emits a `LintInfo` alongside the generated `Lint` which are gathered into `declared_lint::LINTS`. The categories/explanations are then read from `declared_lint::LINTS` at runtime

The removal of `src/docs` is split into a separate commit to be more easily ignored

It is slightly slower though, adding a bit under a second to build time. Less noticeable in full builds or with a slower linker (benchmark uses mold)

```bash
hyperfine --warmup 2 \
    --parameter-list commit "declare-proc-macro,master" \
    --command-name "{commit}" \
    --setup "git checkout {commit}" \
    --prepare "touch clippy_lints/src/lib.rs" \
    "cargo build"
```
```
Benchmark 1: declare-proc-macro
  Time (mean ± σ):     10.731 s ±  0.154 s    [User: 7.739 s, System: 1.791 s]
  Range (min … max):   10.598 s … 11.125 s    10 runs

Benchmark 2: master
  Time (mean ± σ):      9.422 s ±  0.094 s    [User: 7.183 s, System: 1.732 s]
  Range (min … max):    9.287 s …  9.624 s    10 runs

Summary
  'master' ran
    1.14 ± 0.02 times faster than 'declare-proc-macro'
```

r? `@flip1995`
cc `@llogiq` for `--explain`

changelog: none

1 files changed, 0 insertions, 26 deletions

diff --git a/src/docs/mut_from_ref.txt b/src/docs/mut_from_ref.txt
deleted file mode 100644
index cc1da12549a..00000000000
--- a/src/docs/mut_from_ref.txt
+++ /dev/null
@@ -1,26 +0,0 @@
-### What it does
-This lint checks for functions that take immutable references and return
-mutable ones. This will not trigger if no unsafe code exists as there
-are multiple safe functions which will do this transformation
-
-To be on the conservative side, if there's at least one mutable
-reference with the output lifetime, this lint will not trigger.
-
-### Why is this bad?
-Creating a mutable reference which can be repeatably derived from an
-immutable reference is unsound as it allows creating multiple live
-mutable references to the same object.
-
-This [error](https://github.com/rust-lang/rust/issues/39465) actually
-lead to an interim Rust release 1.15.1.
-
-### Known problems
-This pattern is used by memory allocators to allow allocating multiple
-objects while returning mutable references to each one. So long as
-different mutable references are returned each time such a function may
-be safe.
-
-### Example
-```
-fn foo(&Foo) -> &mut Bar { .. }
-```
\ No newline at end of file