docs: add reduced-operator GCC 9 workaround

[zcxGGmu]

Summary

• Add a troubleshooting note for reduced-operator custom builds on Ubuntu 20.04 with GCC 9.
• Recommend using a newer compiler such as gcc-10/g++-10 if the build fails on an unused-parameter warning treated as an error.

Fixes #13898

Test Plan

• python focused check for the Ubuntu 20.04 / GCC 9 troubleshooting note
• git diff --check
• diff secret/conflict scan

[azure-pipelines]

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