Remove benchmark section from root README

[AntonLem]

Motivation

• The JMH sorting benchmark is highly implementation-specific and not relevant to most users visiting the project landing page, so the brief paragraph in the root README.md was removed while preserving the full benchmark documentation in docs/benchmark.md.

Description

• Removed the "Sorting performance benchmark (JMH)" section from the repository root README.md and left docs/benchmark.md unchanged so detailed benchmark instructions remain available.

Testing

• Ran the full build and test suite with JAVA_HOME=/root/.local/share/mise/installs/java/21.0.2 MAVEN_ARGS= mvn -B -ntp verify, which completed successfully (BUILD SUCCESS) and the automated tests passed.

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Codex Task