[Documentation]

Nobody has claimed this yet.

### Describe the documentation issue

I have discovered that onnxruntime will not compile on ubuntu 20.04 with the default gcc version 9.4.0 when using the `--include_ops_by_config` build flag. For me, this results in a "parameter is set but not used" error from [copy.h:231](https://github.com/microsoft/onnxruntime/blob/81c2defd3b7081041db7ca27e3a28351cf039083/onnxruntime/core/framework/copy.h#L231) of `thread_pool` and `dst_offset` when enabled evaluates to false.

The solution is very easy, simply install `gcc-10` and `g++-10` and invoke the build script with `CC=gcc-10 CXX=g++-10`.

[I believe this is the relevant gcc issue.](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81676)

I think it would be great to add a note to the documentation under the Reduce Operator Kernels section to use gcc-10 on Ubuntu 20.04, in order to hopefully save someone a few hours of time. Or, even if a note is not added, perhaps we can just close this issue and let it serve as documentation for anyone who really comes looking.

### Page / URL

https://onnxruntime.ai/docs/build/custom.html#reduce-operator-kernels

Open the Reduce Operator Kernels section at the linked custom build documentation page and review how build prerequisites and flags are described. Add a note covering Ubuntu 20.04 with the default GCC version, the gcc-10/g++-10 packages, and the CC/CXX overrides; the documentation should clearly warn users how to avoid this compilation failure.

Written by the indexing model from the issue text.