docs: fix broken Extended Usage navigation link in README

[codewithfourtix]

Description

Fixed a broken navigation link in the README.

The "Extended Usage" link in the top navigation bar was pointing to
#extended_usage which does not match the actual heading anchor.
Updated it to #boosting-performance-and-extended-applications which
correctly resolves to the "Boosting Performance and Extended Applications"
section.

Type of Change

• Documentation update

What changed

• Fixed: #extended_usage → #boosting-performance-and-extended-applications

What did not change

• No code, config, or functionality touched

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