[docs] Fix broken systemperf install path in perfstress_tests.md

## Summary

Reviewed all 55 markdown files under `doc/` for inconsistencies with the current source code and repository structure. One broken path was found and fixed.

## Change

**File:** `doc/dev/perfstress_tests.md`

**Before:**
```
pip install -e azure-sdk-for-python/tools/devtools_testutils[systemperf]
```

**After:**
```
pip install -e azure-sdk-for-python/eng/tools/azure-sdk-tools[systemperf]
```

**Why:** The `devtools_testutils` package was relocated from `tools/devtools_testutils/` to `eng/tools/azure-sdk-tools/`. The old path no longer exists in the repository, so anyone following this instruction would get a "no such file or directory" error. The `systemperf` optional dependency is defined in `eng/tools/azure-sdk-tools/pyproject.toml`, confirming the correct path.

## Other files checked (no changes needed)

- **`tox` references:** None found — docs already use `azpysdk` throughout.
- **`setup.py` references:** All contextually appropriate — appear only in legacy-package notes alongside `pyproject.toml` as the modern alternative.
- **Broken internal links:** None — all image and `.md` cross-references resolve correctly.
- **Contradictions between docs:** None found.
- **Stale package/path references:** Only the one fixed above.

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---

> [!NOTE]
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> The changes have been pushed to branch `docs/fix-perfstress-install-path-201f0f86227b27d5`.
>
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Show patch (30 lines)

```diff
From afafceb3699f15ce7cf8248fcee9753b936554c3 Mon Sep 17 00:00:00 2001
From: "github-actions[bot]"
Date: Mon, 24 Aug 2026 05:03:18 +0000
Subject: [PATCH] Fix broken install path in perfstress_tests.md

Update systemperf extra install path from tools/devtools_testutils to
eng/tools/azure-sdk-tools, which is the current location of the package.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
doc/dev/perfstress_tests.md | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/dev/perfstress_tests.md b/doc/dev/perfstress_tests.md
index c0f7fa85..fe3ded40 100644
--- a/doc/dev/perfstress_tests.md
+++ b/doc/dev/perfstress_tests.md
@@ -486,7 +486,7 @@ If your tests are not being discovered, run the `perfstressdebug` command instea
The system tests are used to test the performance of the Python HTTP layers exclusive of the Azure SDK in order to set a performance benchmark.
In order to run these, you will need a Python environment with `systemperf` flavour of `devtools_testutils` installed. Installing to a fresh Python environment is recommended.
```cmd
-(env) ~/> pip install -e azure-sdk-for-python/tools/devtools_testutils[systemperf]
+(env) ~/> pip install -e azure-sdk-for-python/eng/tools/azure-sdk-tools[systemperf]
```
Once these dependencies are installed, the `systemperf` command can be run directly to list the available tests:
```cmd
--
2.54.0

```

doc/dev/perfstress_tests.md を開き、systemperf のインストール コマンドと eng/tools/azure-sdk-tools/pyproject.toml にあるパッケージの場所を比較します。記載されているパスがリポジトリの構造と一致していること、およびインストール後に systemperf コマンドを実行できることを確認します。tools/devtools_testutils の古いパスが記載されなくなれば完了です。

索引モデルが issue の本文から書いたものです。