Summary: Link to the README page built with the rest of the documentation, a future-proof solution. (cherry picked from FBD33357870)
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)