[BOLT][DOCS] Build doxygen documentation

Summary:
Added doxygen configuration files and CMake directives, copy-pasta from flang.

```cmake -G Ninja ../llvm-project/llvm \
  -DLLVM_ENABLE_PROJECTS="bolt" \
  -DBOLT_INCLUDE_DOCS=YES \
  -DLLVM_ENABLE_DOXYGEN=YES
ninja doxygen-bolt
```

(cherry picked from FBD33303249)