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, 27 deletions

diff --git a/src/docs/empty_loop.txt b/src/docs/empty_loop.txt
deleted file mode 100644
index fea49a74d04..00000000000
--- a/src/docs/empty_loop.txt
+++ /dev/null
@@ -1,27 +0,0 @@
-### What it does
-Checks for empty `loop` expressions.
-
-### Why is this bad?
-These busy loops burn CPU cycles without doing
-anything. It is _almost always_ a better idea to `panic!` than to have
-a busy loop.
-
-If panicking isn't possible, think of the environment and either:
-  - block on something
-  - sleep the thread for some microseconds
-  - yield or pause the thread
-
-For `std` targets, this can be done with
-[`std::thread::sleep`](https://doc.rust-lang.org/std/thread/fn.sleep.html)
-or [`std::thread::yield_now`](https://doc.rust-lang.org/std/thread/fn.yield_now.html).
-
-For `no_std` targets, doing this is more complicated, especially because
-`#[panic_handler]`s can't panic. To stop/pause the thread, you will
-probably need to invoke some target-specific intrinsic. Examples include:
-  - [`x86_64::instructions::hlt`](https://docs.rs/x86_64/0.12.2/x86_64/instructions/fn.hlt.html)
-  - [`cortex_m::asm::wfi`](https://docs.rs/cortex-m/0.6.3/cortex_m/asm/fn.wfi.html)
-
-### Example
-```
-loop {}
-```
\ No newline at end of file