Fix dead benchmarks link in documentation

The benchmarks page on GitHub Pages no longer exists. Update the
documentation to point users to the in-repo benchmark directories
instead.

Fixes #4163

Signed-off-by: rohansood10 <rohansood10@users.noreply.github.com>

Author: rohansood10

docs/performance/benchmarks.rst

@@ -1,4 +1,7 @@
 Performance Tests - Benchmarks
 ==============================
 
-Click `here <https://open-telemetry.github.io/opentelemetry-python-contrib/benchmarks/index.html>`_ to view the latest performance benchmarks for packages in this repo.
+Performance benchmarks for individual packages can be found within each package's directory
+(e.g., ``propagator/opentelemetry-propagator-aws-xray/benchmarks/``).
+
+To run benchmarks locally, install the package with its benchmark dependencies and use ``pytest --benchmark-only``.