docs: remove dead STRESS.md link from stress-testing page

[ayoubil]

Fixes #5261.

The stress-testing page contained a 'How To Reproduce' section pointing to STRESS.md at the repo root, but no such file exists — the link returns 404. Searching the repo for any STRESS* file confirms the doc was never extracted to a separate file (only docs/deployment/stress-testing.mdx exists).

Reading the surrounding context, the section was redundant: the rest of stress-testing.mdx already contains the reproduction details ("Performance Testing", "Testing Scenarios", setup expectations per volume tier).

Change

Removes the 4-line dead-link section:

-## How To Reproduce
-
-To reproduce the stress testing scenarios mentioned above, please refer to the [STRESS.md](https://github.com/keephq/keep/blob/main/STRESS.md) file in Keep's repository. This document provides step-by-step instructions on how to set up, run, and measure the performance of Keep under different load conditions.
-

Why removal vs. relink

There's no equivalent stress-test scripts directory in the repo. If the intention is to expose the actual benchmarking harness (e.g., scripts under tests/load/ or similar), that's a separate task — beyond what #5261 reports. This PR addresses only the broken-link symptom.

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