Improve build and run instructions for hpolytope-volume example

[Akash504-ai]

Summary

This pull request improves the README documentation for the
examples/hpolytope-volume example in volesti.

What was changed

• Added a short explanation of the purpose of the example
• Updated build instructions to use an out-of-source build, which is the
  recommended CMake workflow
• Documented how to optionally specify a local lp_solve installation
• Added clear instructions for running the example after compilation

Motivation

The previous instructions required manual configuration and could be
confusing for new users. These changes make the example easier to build
and run, especially for first-time users of volesti, without changing
any existing code or behavior.

[vissarion]

Thanks for your PR.