[clang] Add Documentation for Execution Results Handling in Clang-Repl (PR #65650)

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

@AaronBallman, we need to install dot/doxygen on this machine as Sphinx requires graphviz. The PR has a link to the discussion and we could not find the bot owners. 

https://github.com/llvm/llvm-project/pull/65650