Document `LIBZSTD_CFLAGS` and `LIBZSTD_LIBS` in `configure.rst`

# Feature or enhancement

Link: https://docs.python.org/3/using/configure.html#options-for-third-party-dependencies

Refs https://github.com/python/cpython/issues/132983
CC @emmatyping 

I have a PR ready.


### Linked PRs
* gh-135505
* gh-135515