Merge bitcoin/bitcoin#28245: doc: use llvm-config for bitcoin-tidy example

d82bb90 doc: use llvm-config for bitcoin-tidy example (fanquake)

Pull request description:

  An LLVM installation will have `llvm-config` available to query for info. Ask it for the `--cmakedir`, and use that in our bitcoin-tidy example, rather than listing multiple different (potential) paths per distro/OS etc.

ACKs for top commit:
  theuni:
    ACK d82bb90.
  jonatack:
    ACK d82bb90
  TheCharlatan:
    Nice, Re-ACK d82bb90

Tree-SHA512: e07e979231f8f000deafce0751bed4b73ff0eff995bec49e90f579c9051cf5859dac5e49554b8219d33b00c81192db979eed98fee1c643a9205ea8babfce2c5d

Author: fanquake

contrib/devtools/bitcoin-tidy/README

@@ -3,6 +3,9 @@
 Example Usage:
 
 ```bash
-cmake -S . -B build -DLLVM_DIR=/path/to/lib/cmake/llvm -DCMAKE_BUILD_TYPE=Release
-make -C build -j$(nproc)
+cmake -S . -B build -DLLVM_DIR=$(llvm-config --cmakedir) -DCMAKE_BUILD_TYPE=Release
+
+cmake --build build -j$(nproc)
+
+cmake --build build --target bitcoin-tidy-tests -j$(nproc)
 ```